Implement a maintenance mode

[joerghoh]

There should be a state indicating a system maintenance. This maintenance mode should have these properties:

• The reported state should be considered similar to CRITICAL, so the system is automatically taken out of service by all connected services monitoring this system.
• It should be reported separately as MAINTENANCE instead of CRITICAL, so it is clear, that it is not a problem, but regular (and planned) maintenance.
• The overall statuspage should indicate it clearly as maintenance.
• It should be possible to set the maintenance mode (switch on and turn off) via Java API, JMX and OSGI configuration.
• When the maintenance mode changes, the system should report it via an OSGI event.

[joerghoh]

Extend the StatusService interface:

• setMaintenanceMode (boolean enabled)
• boolean getMaintenanceMode();