Change the max hosts per subsystem and add a global host limit

[gbregman]

We need to change the default value for the maximal number of hosts per subsystem to 128. And to add a field to limit the total number of hosts. the default value for the total number of hosts is 2048.

[caroav]

Let's do it a power of 2 , i.e. 2048 will be the default. Both should be configurable.

[gbregman]

@caroav changed to 2048. The existing field "max_hosts_per_subsystem" is already configurable in the conf file and in cephadm spec file.