qa/tasks/cephadm: reject roleless clusters that define mon roles

Combining 'roleless: true' with explicit mon roles fabricates host-named
mons on top of the role-named ones and lets cephadm bootstrap's default
mon service placement race with the task's explicit placement, deploying
a host-named mon on ports assigned to a role-named mon. The role-named
mon then crash-loops on 'Address already in use' and the task times out
180s later with an unhelpful MaxWhileTries error. Fail fast with a clear
message instead.

Fixes: https://tracker.ceph.com/issues/79685
Signed-off-by: David Galloway <david.galloway@ibm.com>