fix(docker-backend): do not start scylla-manager installation

If scylla-manager is not disabled in a test config when running a test on docker
backend, the test will try to install the manager and eventually fail (this is
due to the fact that SCT installs scylla-manager server on monitor node and docker
backend doesn't support dedicated monitoring node docker instance).

The change adds a check for this situation, to be able to fail fast and
do not continue with manager installation od docker backend.

Closes: #9028

sdcm/cluster.py

@@ -5497,6 +5497,8 @@ def node_startup(self, node, **kwargs):  # pylint: disable=unused-argument
         pass
 
     def install_scylla_manager(self, node):
+        if node.is_docker():
+            raise ScyllaManagerError("Scylla Manager is not supported in docker environment")
         if self.params.get("scylla_repo_m"):
             scylla_repo = self.params.get("scylla_repo_m")
         else: