New ssh commands module and commands mock function

Few remarks: 
`enable_node` runs every time the components run. (well, when the node is reachable) This runs all the ssh virsh things every time? Maybe add a simple check if the node is already enabled, if easily possible?
Another thing I see: the node is added regardless the reachability, which is ok, but it also gets enabled, also when the virsh things are not run. So as we discussed, when the node comes  up, everything seems ok for opennebula, but it will give an error when trying to use it. I guess one of these actions should be taken: a) document it, b) add host in disabled state, and enable only when host is reachable in the `enable_node` function;-), or c) add some check in opennebula monitoring that it only activates hosts for usage when the ceph cluster is reachable, like you said. 
@stdweird ?