bug 760 skip get-ssh-public-key,generate-ssh-key,verify-ssh-credentials...

bug 760 skip get-ssh-public-key,generate-ssh-key,verify-ssh-credentials methods for non ssh-layered proxy-charms

Change-Id: I597c66df8c56662f15f8949e4a10aa73f7f5d4ac
Signed-off-by: tierno <alfonso.tiernosepulveda@telefonica.com>