Add credential parameters to the GCE modules.

In order to simplify the workflow with the GCE modules, it's now
possible to add the parameters and project name as arguments to the
various GCE modules.

The inventory plugin also returns the IP of the host in
`ansible_ssh_host` so that you don't have to specify IPs into the
inventory file.

Some update to the documentation are also added.

Closes #5583.
Merge pull request #6265 from franckcuny/gce-fixes

Add credential parameters to the GCE modules.