Ticket #83: Allow to automatically (de)activate targets

- Add a new 'active' option for targets in model file,
  handled at start|tune|mount time.
  Possible values are (default 'yes'):
    - 'yes'     : activate the target on MDT and clients
    - 'nocreate': deactivate the target on MDT
    - 'no'      : deactivate the target on MDT and clients
    - 'manual'  : ignore this target when tuning filesystem
- Add support for this new option in 'shine update'

Change-Id: I2225e5ddb9f72d76ff876d513b7123bcfa8aaac8