don't ask for cinder vg data if present in answers file

This patch introduces a new configs group for cinder's plugin, that
holds the cinder's VG related parameters and will be promted to the user
when those parameters are not present.

patch amended by derekh
o removing to paramaters, the defaults are fine
o checking if the volume group exists befor tryingt to create it
o rebased

Change-Id: If9883885982da0c61f113ea88a35dce0cd2f4681

packstack/modules/ospluginutils.py

@@ -72,13 +72,13 @@ def gethostlist(CONF):
     for key, value in CONF.items():
         if key.endswith("_HOST"):
             value = value.split('/')[0]
-            if value not in hosts:
+            if value not in hosts and value:
                 hosts.append(value)
         if key.endswith("_HOSTS"):
             for host in value.split(","):
                 host = host.strip()
                 host = host.split('/')[0]
-                if host not in hosts:
+                if host not in hosts and host:
                     hosts.append(host)
     return hosts