Extract provider tree functional tests into new file.

test_servers.py was already far too big, so move all tests relating to
the provider tree out into test_servers_provider_tree.py, as discussed
on IRC:

http://eavesdrop.openstack.org/irclogs/%23openstack-nova/%23openstack-nova.2019-05-20.log.html#t2019-05-20T15:44:22

Change-Id: Ibbd57a78b23efe368d92aa6ba0f9ee2d31728e5a

Author: Adam Spiers