Add tests for server metadata

* Raise exception while passing too many metadata items during
set server metadata
* Raise exception while passing too many metadata items during
update server metadata
* Passing an empty body during update server metadata preserves
original metadata items
* Make exception asserts consistent in all test cases

Partially implements bp missing-tempest-tests

Change-Id: I66d5102ddda10072943c454578eb445a3a480b1c