Nailgun plugins support

Basic version

Not solved issues:
- raise or ignore if plugin tasks fail;
- puppet runs using puppet or shell agent.

Change-Id: I1af308ff802fd7911f617168624f994dd69c563b
Implements: blueprint cinder-neutron-plugins-in-fuel
Improvements for Nailgun hooks

- implemenent raise workflow similar with critical
  roles (fail_deployment_on_error). Default - true;
- show detailed message for user in case of fail
  nailgun hook;
- implement support of cwd for execute_shell_command
  mclient agent and nailgun hooks;
- also refactring and tests.

Change-Id: I4458e7e66f62f6bf54509c3820e2ed0b69ea6315
Implements: blueprint cinder-neutron-plugins-in-fuel