[Conformance][templates] templateinstance impersonation tests should pass impersonation update tests

[mfojtik]

/tmp/openshift/build-rpm-release/tito/rpmbuild-originTkCUS7/BUILD/origin-3.7.0/_output/local/go/src/github.com/openshift/origin/test/extended/templates/templateinstance_impersonation.go:180
Expected error:
    <*errors.StatusError | 0xc4213f1880>: {
        ErrStatus: {
            TypeMeta: {Kind: "", APIVersion: ""},
            ListMeta: {SelfLink: "", ResourceVersion: ""},
            Status: "Failure",
            Message: "groups \"impersonategroup\" already exists",
            Reason: "AlreadyExists",
            Details: {
                Name: "impersonategroup",
                Group: "",
                Kind: "groups",
                UID: "",
                Causes: nil,
                RetryAfterSeconds: 0,
            },
            Code: 409,
        },
    }
    groups "impersonategroup" already exists
not to have occurred

Seen: https://openshift-gce-devel.appspot.com/build/origin-ci-test/pr-logs/pull/15624/test_pull_request_origin_extended_conformance_gce/6794/#-conformancetemplates-templateinstance-impersonation-tests-should-pass-impersonation-update-tests