Make ssh key type configurable

[alxwr]

PR progress checklist (to be filled in by reviewers)

• Changes to documentation are appropriate (or tick if not required)
• Changes to tests are appropriate (or tick if not required)
• Reviews completed

---

What type of PR is this?

Primary type

• [build] Changes related to the build system
• [chore] Changes to the build process or auxiliary tools and libraries such as documentation generation
• [ci] Changes to the continuous integration configuration
• [feat] A new feature
• [fix] A bug fix
• [perf] A code change that improves performance
• [refactor] A code change that neither fixes a bug nor adds a feature
• [revert] A change used to revert a previous commit
• [style] Changes that do not affect the meaning of the code (white-space, formatting, missing semi-colons, etc.)

Secondary type

• [docs] Documentation changes
• [test] Adding missing or correcting existing tests

Does this PR introduce a BREAKING CHANGE?

No. Once generated, the admin's SSH key stays untouched.

Related issues and/or pull requests

none

Describe the changes you're proposing

ssh-rsa is getting phased out. This MR uses ssh-ed25519 as the new standard.

https://security.stackexchange.com/questions/226131/openssh-declares-ssh-rsa-deprecated-what-do-i-do-next

Pillar / config required to test the proposed changes

---
gitolite:
  ssh_admin_pubkey_type: 'ed25519'

Debug log showing how the proposed changes work

myhost:
----------
          ID: generate_git_admin_key
    Function: cmd.run
        Name: ssh-keygen -t ed25519 -N '' -f /home/git-admin/.ssh/id_ed25519
      Result: True
     Comment: Command "ssh-keygen -t ed25519 -N '' -f /home/git-admin/.ssh/id_ed25519" run
     Started: 13:34:58.844644
    Duration: 906.05 ms
     Changes:   
              ----------
              pid:
                  37388
              retcode:
                  0
              stderr:
              stdout:
                  Generating public/private ed25519 key pair.
                  Your identification has been saved in /home/git-admin/.ssh/id_ed25519
                  Your public key has been saved in /home/git-admin/.ssh/id_ed25519.pub
                  The key fingerprint is:
                  [...]
----------
          ID: /home/git/gitolite-admin.pub
    Function: file.copy
      Result: True
     Comment: Copied "/home/git-admin/.ssh/id_ed25519.pub" to "/home/git/gitolite-admin.pub"
     Started: 13:34:59.751475
    Duration: 5.507 ms
     Changes:   
              ----------
              /home/git/gitolite-admin.pub:
                  /home/git-admin/.ssh/id_ed25519.pub
              user:
                  git
----------
          ID: setup_gitolite_git
    Function: cmd.run
        Name: /home/git/gitolite/src/gitolite setup -pk /home/git/gitolite-admin.pub
      Result: True
     Comment: Command "/home/git/gitolite/src/gitolite setup -pk /home/git/gitolite-admin.pub" run
     Started: 13:34:59.789546
    Duration: 1570.74 ms
     Changes:   
              ----------
              pid:
                  37400
              retcode:
                  0
              stderr:
              stdout:
----------
          ID: clone_admin_repo_git
    Function: git.latest
        Name: git@myhost:gitolite-admin.git
      Result: True
     Comment: git@myhost:gitolite-admin.git was fetched, resulting in updated refs. Repository was fast-forwarded to origin/master (228402b).
     Started: 13:35:01.361495
    Duration: 1991.768 ms
     Changes:   
              ----------
              revision:
                  ----------
                  new:
                      [...]
                  old:
                      [...]

Summary for myhost
-------------
Succeeded: 25 (changed=4)
Failed:     0
-------------
Total states run:     25
Total run time:    6.571 s

Documentation checklist

• Updated the README (e.g. Available states).
• Updated pillar.example.

Testing checklist

• Included in Kitchen (i.e. under state_top).
• Covered by new/existing tests (e.g. InSpec, Serverspec, etc.).
• Updated the relevant test pillar.

Additional context