qa/tasks: add error condition to exec functions

Looking at the code that expands `all-roles` and `all-hosts` there's no
proper error checking for when these values appear but there are >1
top-level roles in the task config. If a user does this it'll fail
but in a somewhat unclear manner. Add a new condition that raises a
clear exception in this case hopefully saving someone future debugging
time.

Signed-off-by: John Mulligan <jmulligan@redhat.com>

qa/tasks/cephadm.py

@@ -1429,6 +1429,10 @@ def _expand_roles(ctx, config):
         a = config['all-hosts']
         roles = teuthology.all_roles(ctx.cluster)
         config = dict((id_, a) for id_ in roles if id_.startswith('host.'))
+    elif 'all-roles' in config or 'all-hosts' in config:
+        raise ValueError(
+            'all-roles/all-hosts may not be combined with any other roles'
+        )
     return config