Make the UseFqdnForRdpIfAvailable engine-config option available

BZ#901415 Introduced a new feature with this engine option.
For convenience reasons we should expose this option within
engine-config

Change-Id: I913ac5463aef9d9bd9f58fdcdf588e7283d2fa99
Signed-off-by: Vinzenz Feenstra <vfeenstr@redhat.com>
Signed-off-by: Frantisek Kobzik <fkobzik@redhat.com>
Bug-Url: https://bugzilla.redhat.com/1028050