Added external_ips parameter to GCE

Allow user to use a static IP.
 The parameter is a list, so user gives a list of IP's name or 'ephemeral' for a new (as in libcloud).

If several instances are created and there are not enough element in ``external_ips``, missing will be 'ephemeral'.

Module will fail if a unknown IP's name is given.

By default the parameter is an empty list, so the module hold its behavior and attach new IP to VM.