Fix for https://wso2.org/jira/browse/MB-850

- If the admin role name is changed from user-mgt.xml this will be reflected in
  isAdminUser(...) check.

- Admin role name is now taken from current users userRealms admin role name
  rather than the hardcoded value "admin".
- everyone role name string is also taken from userRealm configuration instead of
  hardcoded value "everyone".