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>