tests: always set vif_driver to mock in PCSDriverTestCase
tests: no need to set volume_driver in PCSDriverTestCase

Volume driver is being set automatically to fake, because
we modify CONF.pcs_volume_drivers.
tests: use _prep_instance_and_vm instead of _prep_plug_vifs
tests: add some checks to spawn testcases

Check that VM exists after spawn and that vif_driver.plug called.
tests: add testcase for get_host_info