blueprint host-aggregates: maintenance operations to host OSAPI exts
blueprint host-aggregates: maintenance operations to host OSAPI exts

This commit introduces sub for a host maintenance_mode option into
the osapi. It is required for the host aggregates work.

This is part of a series of commits that have started with change:
https://review.openstack.org/#change,3035

Change-Id: Ib462ef9ad6b641bf1083cc176dfba6645020ccec

n0ano/ganttclient