Add tests for binding extended attributes for ports

Add the following tests:
   1. Create a port from an administrative user specifying
   binding extended attributes;
   2. Update a port from an administrative user specifying
   binding extended attributes;
   3. List ports from an administrative user and confirm
   binding extended attributes are returned;
   4. Show a port from an administrative user and confirm
   binding extended attributes are returned;

Make a small refactoring of port tests and add more checks
for port attributes in test_show_port

Change-Id: Id5fbf5654e6abc3749b765585d2e1c8f3c63f3e1
Merge "Add tests for binding extended attributes for ports"
Merge branch 'master' of https://github.com/openstack/tempest