disabled assignment, in edit user modal page, of a membership between root (fake) role and user (possible if no other role exists)

git-svn-id: https://svn.apache.org/repos/asf/syncope/branches/1_1_X@1604174 13f79535-47bb-0310-9956-ffa450edef68
[SYNCOPE-512] merge with branch

git-svn-id: https://svn.apache.org/repos/asf/syncope/trunk@1604178 13f79535-47bb-0310-9956-ffa450edef68