Support plugin machine-specific setting

[arabnejad]

it would be great if we can have a machine-specific setting for each plugin.

the current hierarchy for loading env variables are :

1. loading defaults env variable from machines.yml
2. Updating (add/override) env variable from machines_user.yml

and with this new support, we have the third level

3. Updating (add/override) env variable, for a specific plugin, from machines_<plugin name>_user.yml if file exists

[arabnejad]

Closed with commit 795cfab