ncm-ceph: add option to gather ssh hosts keys automatically

Fix for issue #213: ssh key gathering

Builds on pr #248, review this pr after #248
 
this gives
```
[root@hyp102 ~]# ssh-keygen -F hyp101
ssh-keygen: /root/.ssh/known_hosts: No such file or directory
```
with missing `known_hosts`, which will end up in your output