Add ssh-keys to plugs for openstack commmand

When importing ssh keys using openstack keypair create
we get Permission denied, running with snappy-debug
suggests to add ssh-key to plugs.

ssh-public-keys is being dropped as ssh-keys interface
also allows access to public keys.

Change-Id: Ib7e6a13da8b34e03d5ad042e5e4862ff14c4dc87

snapcraft.yaml

@@ -30,7 +30,7 @@ apps:
       - network
       - home
       - dot-config-openstack-clouds-yaml
-      - ssh-public-keys
+      - ssh-keys
   glance:
     command: bin/glance
     plugs: