Adds link_roles to ReportPortal in plugin registry
Adds link_roles to ReportPortal in plugin registry

That will save the users from the need to manually create a symbolic
link to the roles directory before being able to run the plugin.
Once the 'link_roles' is set to 'true' the link will be created
automatically when adding the plugin.

RHOSINFRA-4141

Change-Id: I0c2eb027e08f1c78881def7f60dbec6df43c72be

redhat-openstack/infrared,redhat-openstack/infrared,redhat-openstack/infrared