qa/tasks: add error condition to exec functions

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>