core: Add network custom properties configuration values

Report support for network custom properties starting from version
3.5, as well as add configuration values for both pre-defined custom
properties and user-defined ones. The user-defined entry is also
created by default in the DB for version 3.5, so that they can be
configured via the engine-config utility.

Change-Id: I90c2f011b82b22a9df2e4558256d450392a13035
Signed-off-by: Lior Vernia <lvernia@redhat.com>