hail: allow overriding cluster data from requests

Currently, it's not easy to generate “fake” IAllocator request files
for hail. As such, testing on simulated clusters is hard to do.

To workaround this, we change hail to also take the ‘-t’ and
‘--simulate’ options, so that we can override the cluster data read
from the request. Note that this will not change the request itself
(so for example an evacuation will need to make sure uses the correct
node names), but it's a step forward in testing hail. The other tools
already can use text files which allow for better flexibility.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Balazs Lecz <leczb@google.com>