Azure disk create should fail if disk already exists
Azure disk create should fail if disk already exists

Addressed other review comments

Signed-off-by: Piyush Nimbalkar <22b9c231e86e73b6822f10a284ad53e606c09e6d@portworx.com>

libopenstorage/openstorage,libopenstorage/openstorage,libopenstorage/openstorage