T6169: Added dry run mode and polling time

• Allow to check OTA upgrades availability

  If the dry run mode is set then just check is the OTA update is available and do not try to download and apply the upgrade with:

  updatectl -u --dry-run

  The log from updatectl started in monitor mode will have new status:

  AUM-Message: 14:45:27.283: Upgrade status: Available

  and the AUM log will contain the new string:

  Feb 21 14:45:27 apertis apertis-update-[888]: Network upgrade is available

• Allow to try the static delta without an upgrade.

  If the --dry-run is used for offline upgrade, the static delta will pass over all checks but upgrade will not started. The output from updatectl started in monitoring mode will contain:

  AUM-Message: 15:32:30.993: Upgrade status: Available

  and the log of AUM:

  Feb 21 15:32:57 apertis apertis-update-[1098]: Static delta is applicable for the system

• Added option changing network updates polling time:

  updatectl --check-network-updates 3600