ceph: nautilus tests run on 14.2.7 instead of latest release

The CI is failing currently in the OSD-on-pv scenario due to a change
in v14.2.8. Until that is resolved, we need to unblock the tests
by running against v14.2.7.

Signed-off-by: Travis Nielsen <tnielsen@redhat.com>

tests/framework/installer/ceph_installer.go

@@ -42,7 +42,7 @@ const (
 	// test with the latest mimic build
 	mimicTestImage = "ceph/ceph:v13"
 	// test with the latest nautilus build
-	nautilusTestImage = "ceph/ceph:v14"
+	nautilusTestImage = "ceph/ceph:v14.2.7"
 	helmChartName     = "local/rook-ceph"
 	helmDeployName    = "rook-ceph"
 	cephOperatorLabel = "app=rook-ceph-operator"