mgr/dashboard: Pool form max size
Currently the pool form max size is determined by "max_size" of the selected rule or the maximum amount of available OSDs. The amount can be wrong if the failure domain of the rule is not OSD.
I'm also currently not sure if "max_size" and "min_size" are useful values to show, at least pools created on a vstart cluster always show the same min and max size values. Please make that sure that those values can still be used safely.