Move optional build dependencies to 'extras'

...and start skipping the relevant tests in 'nova.tests.unit.virt' if
these dependencies are not present. Thanks to hacking rules N311 and
N312, which prevent virt drivers using code from other virt drivers,
simply skipping the unit tests for a virt driver module is enough to
ensure we never load those modules.

This means users that want to use the powervm driver can install the
required dependencies using e.g. 'pip install .[powervm]', and packagers
can choose to skip packaging a dependency safe in the knowledge that the
relevant tests will be skipped.

Change-Id: I3787f9afd78cd0a7c7feb4dfe1bcb21437b5a128
Signed-off-by: Stephen Finucane <stephenfin@redhat.com>

test-requirements.txt

@@ -23,6 +23,3 @@ testtools>=2.5.0 # MIT
 bandit>=1.1.0 # Apache-2.0
 gabbi>=1.35.0 # Apache-2.0
 wsgi-intercept>=1.7.0 # MIT License
-
-# vmwareapi driver specific dependencies
-oslo.vmware>=3.6.0 # Apache-2.0