Rename scenario.manager._create_port to create_port

This is to:
1. Rename scenario.manager._create_port to create_port,
   because it is used by testcases outside the file.
2. Remove namestart parameter from _create_port, because
   name is not important for scenario testcases and to
   use class name as the prefix of port name will be more
   indicative.
3. Remove self.assertIsNotNone(result, 'Unable to allocate port')
   because when using expect_empty_body = False, it's impossible
   to get a None result.
   http://git.openstack.org/cgit/openstack/tempest/tree/tempest/lib/services/network/base.py#n66

Change-Id: Ia1be5af5b6be33e67c5a1256208c0272ae109b39
2 files changed