Support to enable/disable the Bugzilla integration in the UI

On the ProjectInfoScreen there is now a dropdown list for the
plugin.its-bugzilla.enabled parameter which offers the values true,
false, enforced and INHERIT. Project owners can change this parameter
and save it. If the Bugzilla integration is enforced by a parent
project the dropdown list is disabled.

Change-Id: I887d5b891fa8e832d7a04fd1dd0208042dfc85c7
Signed-off-by: Edwin Kempin <edwin.kempin@sap.com>