Fix bug 1542 to allow juju to add Azure AKS

Storage classes in Azure might have no metadata annotations. This
change takes into account that situation.

Change-Id: I4f7151d2481b41fd8434e0d95ea5d1a156b68309
Signed-off-by: garciadeblas <gerardo.garciadeblas@telefonica.com>
(cherry picked from commit e77a91e5)