test_cephfs_shell: set `colors` to Never for cephfs-shell
Originally, the plan was to use setUpClass and tearDownClass for tests1 but I missed pushing that modification before merge. Even if I were push that modification in time, it still would be incorrect because setUpClass would require access to mon_manager.raw_cluster_cmd which can't happen unless cephfs_test_case.CephFSTestCase.setUp2 runs. Therefore, colors has to be set and unset in TestCephFSShell.setUp instead of setUpClass.
Related to this TODO3.