Add ssh identity support

[JasonDictos]

This adds the ssh identity support to the concourse yaml

Original work: #331

Tests pass locally however I cannot determine why the concourse-ci/status failed

[xtremerui]

Hi @JasonDictos
here are the failure logs:

#26 186.8 • Failure [7.431 seconds]

#26 186.8 Out

#26 186.8 github.com/concourse/docker-image-resource/tests/out_test.go:17

#26 186.8   When specifying ssh_identity file

#26 186.8   github.com/concourse/docker-image-resource/tests/out_test.go:487

#26 186.8     should set ssh args [It]

#26 186.8     github.com/concourse/docker-image-resource/tests/out_test.go:488

#26 186.8 

#26 186.8     Got stuck at:

#26 186.8         DOCKERD: --data-root /scratch/docker --mtu 1460 --max-concurrent-downloads=3 --max-concurrent-uploads=3

#26 186.8         waiting for docker to come up...

#26 186.8         Agent pid 3351

#26 186.8         @@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@

#26 186.8         @         WARNING: UNPROTECTED PRIVATE KEY FILE!          @

#26 186.8         @@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@

#26 186.8         Permissions 0644 for '/docker-image-resource/tests/fixtures/ssh_identity' are too open.

#26 186.8         It is required that your private key files are NOT accessible by others.

#26 186.8         This private key will be ignored.

#26 186.8         unset SSH_AUTH_SOCK;

#26 186.8         unset SSH_AGENT_PID;

#26 186.8         echo Agent pid 3351 killed;

#26 186.8         

#26 186.8     Waiting for:

#26 186.8         DOCKER ARG: --ssh

#26 186.8 

#26 186.8     github.com/concourse/docker-image-resource/tests/out_test.go:500