Added optional GS roles to GF groups matching, with usage of roles to filter requests instead of username

Two new options for GeofenceAccessManager (Geoserver probe):
 * useRolesToFilter: true if the probe should build RuleFilter using user roles instead of its username (defaults to false)
 * acceptedRoles: comma delimited list of mutually exclusive roles Geoserver roles that are accepted as Geofence group (allows limiting the filter role / group to one, since currently RuleFilter doesn't allow multiple groups). Always specify a list of mutually exclusive roles, for the filter to be functional.Example: 

acceptedRoles=ROLE_BASIC, ROLE_MEDIUM, ROLE_ADMIN