container platform cleanup and fixes

[alfredodeza]

This PR outgrew the initial intent of just adding docs and tests:

• Fixes https://github.com/ceph/ceph-medic/issues/90
• Adds a compat shim for Python3 imports
• Fixes a lot of Deprecation Warnings on Python 3
• Changes how configuration is set and accessed (via attributes instead of dict)
• Improves the padding for long hosts when reporting results

• Add/improve the documentation for container platform support

   $ ceph-medic check
   Host: rook-ceph-mgr-a-65445fd5f9-67bqm          connection: [failed     ]          
   Host: rook-ceph-osd-0-84dfcbbf7-gtj79           connection: [failed     ]          
   Host: rook-ceph-mon-a-7f94995d9c-qwwsb          connection: [connected  ]          
   Host: rook-ceph-mon-b-958cf7785-2cstl           connection: [connected  ]               
   Host: rook-ceph-mon-c-75b68cb886-n6bnt          connection: [connected  ]               
   Collection completed with some failed connections                                                                      
  
   =======================  Starting remote check session  ========================
   Version:    1.0.4    Cluster Name: "ceph"
   Connection: openshift
   Total hosts: [5]
   OSDs:    0    MONs:    3     Clients:    0
   MDSs:    0    RGWs:    0     MGRs:       0
  
   ================================================================================
  
   ------------ mons ------------
    rook-ceph-mon-a-7f94995d9c-qwwsb                                      
    rook-ceph-mon-b-958cf7785-2cstl                                       
    rook-ceph-mon-c-75b68cb886-n6bnt                                      
  
   -------- Failed Nodes --------
    rook-ceph-mgr-a-65445fd5f9-67bqm                                      
     Remote connection failed while testing connection:
       error: unable to upgrade connection: container not found ("mgr")
    rook-ceph-osd-0-84dfcbbf7-gtj79                                       
     Remote connection failed while testing connection:
       error: unable to upgrade connection: container not found ("osd")
  
   39 passed, on 3 hosts