Poll on update server

Create a GMainLoop to poll update server to retrieve update information.
Update poll sleep timeout based on time sent by the update server.

Improve AhaHawkbitBase.aha_hawkbit_base_update() to return a gboolean:
it returns False if it is unable to retrieve base info from the update
server, True otherwise.
Check json parsing in AhaHawkbitBase.get_uri().

Signed-off-by: Frédéric Danis <frederic.danis@collabora.com>