high: ui_maintenance: Add maintenance sublevel (bnc#899234)

This adds a new sublevel: maintenance

Commands in this sublevel are only valid when the cluster is in
maintenance mode, or the affected resource is in maintenance mode.

This patch also adds two commands in this sublevel:

 - action: invoke a resource action directly using the agent
 - actionssh: invoke a resource action on multiple nodes using pssh
medium: rsctest: Add basic support for systemd services
Fixes #61
Merge pull request #62 from icclab/master

Fixes #61
dev: ui_maintenance: Removed dead code
medium: ui_maintenance: Combine action and actionssh into a single command
low: rsctest: Better error message for unsupported action
Merge pull request #60 from krig/maintenance-level

high: ui_maintenance: Add maintenance sublevel (bnc#899234)