Raise exception if both port and fixed-ip are in requested networks.

When both fixed-ip and port (uuid) are specified as part of the
request during server creation, the user specified fixed-ip must
be ignored because it has already been allocated for the port. For
compute osapi v3, we raise a BadRequest Exception to ensure the user
understands why he/she is not getting what he/she asks for.

This patch also address some minor nits in other error messages.

Fixed bug #1190777

Change-Id: Iaf606637a8acf9d20fc6cae0bed5c9a3ea0bf611
Merge "Raise exception if both port and fixed-ip are in requested networks."