API v2: mprove validation of post/put, rename few attributes

API v2: mprove validation of post/put, rename few attributes

bug #1012438

Additional work for bp v2-api-melange-integration

- rename few attributes:
  op_state -> status
  additional_routes -> additional_host_routes
  prefix -> cidr

- expand request body validation to indicate whether fields can be
specified during create and or update.
- add test cases to validate defaults, and input validation.

- update db_base_plugin_v2 to generate gateway_ip for subnet and mac
for port when unspecified.

- validate that tenant-id is only specified in req by admin users

- automatically set tenant-id based on request.context.tenant_id if needed

- enable port tests in test_db_plugin.py

Change-Id: If7f5101e4974a6ef93ff8a1d945f8642dd21b16e