Move tests test_update_* to separate class

Moved test_update_* in nova.tests.api.openstack.compute.plugins.v3.test_servers
and nova.tests.api.openstack.compute.test_servers to separate classes.

This commit is the first step in refactoring these files.
It will be followed by a commit which will remove duplicated code.

bp nova-tests-code-duplication

Change-Id: Ia78022d32962b846c62afc53ebb70fe17dda7931
Merge "Move tests test_update_* to separate class"