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".
Fixed taking the admin role name from current user store configuration instead of taking
admin name from bootstrap user configuration to check whether a user has admin
privileges.
Merge pull request #54 from Asitha/mb-850

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