Add block volume extension for os-vol-host-attr

[kristian-lesko]

The os-vol-host-attr is returned by OpenStack API;
let's add an extension to allow using this value.

Fixes #2211.

[theopenlab-ci]

Build succeeded.

• gophercloud-unittest : SUCCESS in 5m 11s
• gophercloud-acceptance-test : SUCCESS in 3h 56m 20s
• gophercloud-acceptance-test-ironic : SUCCESS in 1h 05m 21s

[jtopjian]

@kristian-lesko Thank you for submitting this.

This might seem overkill for a single field, but technically os-vol-host-attr:host is an API extension as defined here: https://github.com/openstack/cinder/blob/393c2e4ad90c05ebf28cc3a2c65811d7e1e0bc18/cinder/api/contrib/volume_host_attribute.py

Instead of adding this field to the main Volume struct, create a dedicated package for it like this one: https://github.com/gophercloud/gophercloud/tree/513734676e6495f6fec60e7aaf1f86f1ce807428/openstack/blockstorage/extensions/volumetenants

Please let me know if you have any questions.

[kristian-lesko]

Thanks for your comment @jtopjian; PR updated per your suggestion.

[theopenlab-ci]

Build succeeded.

• gophercloud-unittest : SUCCESS in 4m 59s
• gophercloud-acceptance-test : SUCCESS in 2h 12m 59s
• gophercloud-acceptance-test-ironic : SUCCESS in 1h 10m 03s

[jtopjian]

LGTM - thank you very much!

[coveralls]

Coverage remained the same at 79.821% when pulling 13c3d46 on kristian-lesko:os-vol-host-attr into 3b820fa on gophercloud:master.