common/install: throttle 1 on unpack task

During the common install tasks, we download and unpack the binary to
install on localhost and eventually upload it to each host (among many
other things). Currently the "unpack" step, which extracts the gzipped
tar archive, is perform N times if there are N hosts in the inventory,
but the target directory (something like
/tmp/node_exporter-linux-arm64/1.8.2) is the same for every host with
the same architecture.

This means that all the unarchive tasks are extracting in parallel in an
unsynchronized manner to the same directory. It a miracle that this
works, but at least usually it like it does, but sometimes tar will
complain about a missing file, failing the install. I've confirmed
locally that this is due to the race described above.

To fix this, we use `throttle: 1` on the unpack task. This means that
the first task (for each architecture) will do the unpack and the other
tasks are effectively no-ops, they are reported as "ok" rather than
changed in the output.

Fixes #457.

roles/_common/tasks/install.yml

@@ -102,6 +102,7 @@
         extra_opts: "{{ _common_binary_unarchive_opts | default(omit, true) }}"
       delegate_to: localhost
       check_mode: false
+      throttle: 1 # see prometheus-community/ansible#457
       when: __common_binary_basename is search('\.zip$|\.tar\.gz$')
 
 - name: "Check existence of binary install dir"