Fix all in Fog::Compute::Servers
Accept public_key and public_key_path when creating GCE server

Earlier, the code forced the user to specify the :public_key_path
option when creating the server. In some cases, the user may want
to pass in the key itself (string), and not a path. This patch
changes the code to use the #public_key method, which will first
check if the key was specified, and only attempt to read the file
if it wasn't specified.
[google] client.images doesn't list google public images
[google|compute] servers.get don't catch errors

* If server is not present in first tested zone an error is raised
  and not catched
Merge pull request #2003 from rvrignaud/gce

[google|compute] all method in Google::Compute::Servers should have filters argument
Merge pull request #2031 from rvrignaud/get_server

[google|compute] in servers.get, catch fog errors
Merge pull request #2020 from rvrignaud/images

[google|compute] client.images doesn't list google public images
Merge pull request #2016 from maginatics/gce_accept_public_key_string

[google|compute] Accept public_key and public_key_path when creating GCE server