Added property `UpdatesPollingTime` for delay between OTA upgrades
attempts in seconds. Default is 300 seconds.
Could be set configuration file as well:
 [Manager]
 UpdatesPollingTime=300

Changing the property is not starting the update itself but reset the
timer to new value if update has been enabled already.

Signed-off-by: Denis Pynkin <denis.pynkin@collabora.com>