Skip TrunkTestMtusJSONBase class tests when provider_vlans is not defined

Related-Prod: PRODX-47802
Change-Id: Ibd918c1e1acea3bdb0287cfd91473d43b84136f1
(cherry picked from commit 258b44547f2a27950bd005e4f99baf823d22e080)
1 file changed