deployment-management: Update apertis-hawkbit-agent doc

Add precision on the uniqueness of 'Controller Id'. Add link to apertis-hawkbit-agent on gitlab. Remove reference to 'Tenant' config as it is not mandatory in the apertis-hawkbit-agent configuration file, only the 'DEFAULT' value is supported and it brings confusion. Add apertis-hawkbit-agent configuration file sample.

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