Make sure to remove configuration files that are no longer needed

[ygalblum]

Make sure the role specific configuration file is removed if the instace no longer has the role

Do the same for the custom configuration file

Fixes: #120

[lanefu]

@ygalblum I feel like there should be a failsafe for for this to avoid an undesired cleanup.

maybe pair with a default var like nomad_allow_purge_config: false

[ygalblum]

@lanefu Done