Refactor bluechictl usage

[engelmi]

This PR does split up the method-unit-actions file - which collects a lot of different operations - and creates multiple new files, grouping the commands semantically.
In addition, it also adds specific usage functions for all commands such as:

$ bluechictl start --help
bluechictl is a convenience CLI tool to interact with bluechi

Description: 
  Start/Stop/Restart/Reload a unit on a node

Usage: 
  bluechictl [start|stop|restart|reload] [nodename] [unitname]


Examples:
  bluechictl start primary interesting.service
  bluechictl stop primary interesting.service
  bluechictl restart primary interesting.service
  bluechictl reload primary interesting.service


$ bluechictl status --help
bluechictl is a convenience CLI tool to interact with bluechi

Description: 
  View status for units and nodes of BlueChi

Usage: 
  bluechictl status [nodename] [unitname] [options]

  If [nodename] and [unitname] are not given, the status of all nodes will be displayed.
  If [unitname] is not given, the status of the specific node will be displayed.

Available options:
  --help         shows this help message
  --watch, -w    continuously watch the status of the node(s) and unit

[coveralls]

coverage: 83.553% (-1.6%) from 85.196%
when pulling 09955e0 on engelmi:refactor-bluechictl-usage
into d54adc0 on eclipse-bluechi:main.

[mwperina]

LGTM