mgr/dashboard: Use right size in pool form

mgr/dashboard: Use right size in pool form

Currently the max size is determined by the number of OSDs, which is
compared with the maximum of the current crush rule.

The problem with that is, that this is wrong for every crush rule that
doesn't have OSDs as failure domain and that don't have the root of the
cluster set as root of the crush rule.

Now the crush map will be used to determine how many failure domains are
really available in the cluster and how many can really be used in the
end. This number now defines the maximum size you can enter.

The crush detail view will now the new attribute usable_size and hide
the redundant information steps, ruleset, type and rule_name.

Fixes: https://tracker.ceph.com/issues/44620
Signed-off-by: Stephan Müller <smueller@suse.com>