create a helper to warn against incompatible hostnames

This PR addresses the problem that sometimes remote hosts may not have a matching `hostname` for the host they are deploying to.

So now we check that when a mon is being created and issue a very clear warning. Example output:

    $ ceph-deploy mon create node1
    [ceph_deploy.mon][DEBUG ] Deploying mon, cluster ceph hosts node1
    [ceph_deploy.mon][DEBUG ] detecting platform for host node1 ...
    [ceph_deploy.mon][INFO  ] distro info: Ubuntu 12.04 precise
    [node1][DEBUG ] determining if provided host has same hostname in remote
    [node1][WARNIN] ********************************************************************************
    [node1][WARNIN] provided hostname must match remote hostname
    [node1][WARNIN] provided hostname: node1
    [node1][WARNIN] remote hostname: localhost
    [node1][WARNIN] monitors may not reach quorum and create-keys will not complete
    [node1][WARNIN] ********************************************************************************
    [node1][DEBUG ] deploying mon to node1
    [node1][DEBUG ] remote hostname: localhost
    [node1][INFO  ] write cluster configuration to /etc/ceph/{cluster}.conf
    [node1][DEBUG ] checking for done path: /var/lib/ceph/mon/ceph-localhost/done
    [node1][INFO  ] create a done file to avoid re-doing the mon deployment
    [node1][INFO  ] create the init path if it does not exist
    [node1][INFO  ] Running command: initctl emit ceph-mon cluster=ceph id=localhost