libvirt: convert encrypted LVM test to avoid DB usage

The EncryptedLvmTestCase test case was depending on the DB setup
for no purpose. This masked a bug whereby the test was relying
on the implicit import of nova.keymgr.conf_key_mgr config opts.
Explicitly import the config opt and remove DB setup. This
improves performance from 1.8s to 0.6s

Partial-bug: #1369516
Change-Id: Icd5c27b3ac6cab5a014fae65904bbbefb5343282
Merge "libvirt: convert encrypted LVM test to avoid DB usage"