nova-dpm cannot create partitions when len(cfg.default.host) > 17

Since [1], nova-dpm stores the cfg.default.host in the partitions name field. Partition creation will fail once the hostname is too long.

Along the DPM API reference, the name field is a string with a max len of 64.
The partition name gets constructed like this

  <Openstack prefix>-<host>-<instance uuid>

OpenStack prefix = 'OpenStack'; len = 9
2x separator = '-'; len = 2
uuid = '0a5cb3b6-ff9f-443c-a8f0-34296513be20'; len = 36

already used: 36 + 2 + 9 = 47
left for hostname: 64-47 = 17

We have the following options:
* Live with that: Document it and have a something in the drivers init method that terminates the n-cpu when the hostname is longer

* Move the hostname to the description:
-> that's how it is done in networking-dpm at the moment. It's using the following fomrat

   CPCSubset=cfg.default.host

Having the additional identifier "CPCSubset=" might make sense to avoid issues where simple hostnames like "foo" or "test" are used which might also be part of the name of some other parition. But as the code [1] is checking for the whole format Prefix-cfg.host-uuid this might not be required!

However we decide, I will adapt the way networking-dpm is handling that accordingly!

Ideas?

[1] https://review.openstack.org/#/c/421611