Do not create dangling symlinks for containerized RHSM

When setting RHSM into the container mode, we are creating symlinks
to /etc/rhsm and /etc/pki/entitlement directories. However, this
creates dangling symlinks if RHSM is not installed or user manually
removes one of these dirs.

If any of these directories is missing, skip other actions and
log the warning. Usually it means that RHSM is not actually used
or installed at all, so in these cases we can do the skip. The
only corner case when system could use RHSM without
/etc/pki/entitlement is when RHSM is configured to put these
certificate on a different path, and we do not support nor cover
such a scenario as we are not scanning the RHSM configuration at
all.

This also solves the problems on systems that does not have RHSM
available at all.

repos/system_upgrade/common/libraries/rhsm.py

@@ -325,6 +325,11 @@ def set_container_mode(context):
     could be affected and the generated repo file in the container could be
     affected as well (e.g. when the release is set, using rhsm, on the host).
 
+    We want to put RHSM into the container mode always when /etc/rhsm and
+    /etc/pki/entitlement directories exists, even when leapp is executed with
+    --no-rhsm option. If any of these directories are missing, skip other
+    actions - most likely RHSM is not installed in such a case.
+
     :param context: An instance of a mounting.IsolatedActions class
     :type context: mounting.IsolatedActions class
     """
@@ -332,6 +337,17 @@ def set_container_mode(context):
         api.current_logger().error('Trying to set RHSM into the container mode'
                                    'on host. Skipping the action.')
         return
+    # TODO(pstodulk): check "rhsm identity" whether system is registered
+    # and the container mode should be required
+    if (not os.path.exists(context.full_path('/etc/rhsm'))
+            or not os.path.exists(context.full_path('/etc/pki/entitlement'))):
+        api.current_logger().warning(
+            'Cannot set the container mode for the subscription-manager as'
+            ' one of required directories is missing. Most likely RHSM is not'
+            ' installed. Skipping other actions.'
+        )
+        return
+
     try:
         context.call(['ln', '-s', '/etc/rhsm', '/etc/rhsm-host'])
         context.call(['ln', '-s', '/etc/pki/entitlement', '/etc/pki/entitlement-host'])