Enable Keycloak Admin for Non-master Realms

Allow the 'KeycloakAdmin' class to instantiate against non-master realms using
an Authorization header for a non-admin user that is granted permissions to
inspect or manage Keycloak admin resources.

Example:

kca = KeycloakAdmin(
        'https://auth.keycloak.local/auth/',
        realm_name='my-realm',
        client_id='admin',
        custom_headers=auth_headers
      )

keycloak/keycloak_admin.py

@@ -1885,12 +1885,16 @@ class KeycloakAdmin:
             if self.user_realm_name:
                 self.realm_name = self.user_realm_name
 
-        self._token = self.keycloak_openid.token(self.username, self.password, grant_type=grant_type)
+        if self.username and self.password:
+            self._token = self.keycloak_openid.token(self.username, self.password, grant_type=grant_type)
 
-        headers = {
-            'Authorization': 'Bearer ' + self.token.get('access_token'),
-            'Content-Type': 'application/json'
-        }
+            headers = {
+                'Authorization': 'Bearer ' + self.token.get('access_token'),
+                'Content-Type': 'application/json'
+            }
+        else:
+            self._token = None
+            headers = {}
 
         if self.custom_headers is not None:
             # merge custom headers to main headers