Add cli commands to interact with plugins

- List all plugins installed
fuel plugins
fuel plugins --list

- Install plugin
fuel plugins --install /tmp/sample.pb
1. Extracts metadata from plugin tar
2. If plugin successfully registered in nailgun
3. Untar all the files in /var/www/nailgun/plugins

- If you want to update plugin when it is already installed
  add --force flag
fuel plugins --install /tmp/sample --force
The procedure is the same as for simple install, but
if plugin already registered - it will try to update
(send PUT request)

- Refactored tests to allow writing unit tests, with mock
  and all that goodness

related to blueprint cinder-neutron-plugins-in-fuel

Change-Id: I642a300abcb9c50ef5039a343064f2ac78e39cd8
Merge "Add cli commands to interact with plugins"