Use automated create and update methods for core network resources

The patch makes use of automatic create and update methods for
networks, subnets, ports. Bulk create method was moved to base client
to get rid of code duplication in json and xml versions.

Change-Id: I0e1295d6c195187d36b3b811a0dd28c5477cd07b
Merge "Use automated create and update methods for core network resources"