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