BASIC21 test: added persistent volume checking

Added check on the presence of the persistent volume in Openstack. Created new keywords in openstack_lib.robot for that.
Fix bug in vnf_lib.robot.

Change-Id: I0b4cc8f81d2f1cddcd90d1664a69dafae23e70cb
Signed-off-by: aguilard <e.dah.tid@telefonica.com>