creates a `pkg` sub-command to install/remove packages on remote hosts

One sub-command (`pkg`) with two flags, as seen from the help menu:

  ceph-deploy pkg --help
  usage: ceph-deploy pkg [-h] [--install [PKGs)]] [--remove [PKG(s]]
                         hosts [hosts ...]
  
  Manage packages on remote hosts.
  
  positional arguments:
    hosts
  
  optional arguments:
    -h, --help          show this help message and exit
    --install [PKG(s)]  Comma-separated package(s) to install
    --remove [PKG(s)]   Comma-separated package(s) to remove

Although very simple, some documentation was provided as well a bunch of tests for the `pkg_managers` module.