Handle ExternalNetworkAttachForbidden exception

When creating an instance and assigning a public network
to it without admin authority, ExternalNetworkAttachForbidden
will be raised. But this exception is not handled in V3 api.

Change-Id: Id70d8eaa704fd82cef1afdb65756d75cb56fbdce
Closes-Bug: #1353949
Merge "Handle ExternalNetworkAttachForbidden exception"