Allow setting tls_priority option

Add tls_priority config option to libvirtd.conf config.

libvirt-4.0.0 supports this functionality out of the box.

To fully comply with PROD-27620 priority string should be set in reclass
to the following string:

  SECURE256:-VERS-ALL:+VERS-TLS1.2:-KX-ALL:+ECDHE-RSA:+ECDHE-ECDSA:\
  -CIPHER-ALL:+AES-256-GCM:+AES-256-CBC:-MAC-ALL:+AEAD:+SHA384

Also, fix some inconsistiences found:

* tests/pillar/compute_single.sls - change qemu's group name to 'nova' as
  'cinder' does not exists.

Change-Id: I56a8c3726e1af274e2d2fffce8dca4501745519e
Related-Prod: PROD-27620
(cherry picked from commit 77d9dacf644b1c8a93dcc35d30d90b046b86c9fa)
diff --git a/tests/pillar/compute_single.sls b/tests/pillar/compute_single.sls
index b8754f6..2d00ef3 100644
--- a/tests/pillar/compute_single.sls
+++ b/tests/pillar/compute_single.sls
@@ -53,6 +53,7 @@
       host: 127.0.0.1
       port: 9292
     network:
+      user: nova
       engine: neutron
       region: RegionOne
       host: 127.0.0.1
@@ -73,7 +74,7 @@
         secret_key: secret
     qemu:
       user: nova
-      group: cinder
+      group: nova
       dynamic_ownership: 1
     consoleauth:
       token_ttl: 600