Make GCE image search list comma separated

This updates the option for the image search project list to use commas (,)
rather than colons (:) to separate entries -- group affiliated projects are
identified with a colon in the GCE system, making them impossible to use in
the previous mode.

This also follows the convention of the upstream gcutil CLI.

Signed-off-by: Daniel Pittman <daniel@rimspace.net>
Support `google_compute_engine` ssh key by default

The upstream gcutils tools default to the `google_compute_engine` ssh key;
this adapts our code to look for that first, and default to it if present.

If it does not exist we fall back to the normal `id_rsa` SSH credentials,
which follows the common practice of the rest of Cloud Provisioner.

Signed-off-by: Daniel Pittman <daniel@rimspace.net>
Merge pull request #122 from daniel-pittman/gce-avoid-colon-in-project-list

Make GCE image search list comma separated
Merge pull request #123 from daniel-pittman/gce-support-google-compute-ssh-key-too

Support `google_compute_engine` ssh key by default
(packaging) Update CHANGELOG, Modulefile for 1.1.1