Add support for flexible accommodations

[Phil Sphicas]

For some tests, it is required to have exactly one pair of agents, and it is desired to place them separate compute nodes, if more than one compute node is available.

There is no way to specify this using the existing accommodation parameters.

This works if you have more than two computes, but fails with only a single compute:

[pair, single_room, compute_nodes: 2]

More generally, under some circumstances it would be useful to request a target number of compute nodes, but to allow fallback to whatever is available.

It would better if we had a knob to allow this type of fallback behavior.

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.openstack.org/623971

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.openstack.org/623971
Committed: https://git.openstack.org/cgit/openstack/shaker/commit/?id=3d1d37b96a2153efbd01d2962bc164acf009e266
Submitter: Zuul
Branch: master

commit 3d1d37b96a2153efbd01d2962bc164acf009e266
Author: Sphicas, Phil (ps3910) <email address hidden>
Date: Sun Dec 9 18:01:31 2018 -0800

    Add support for best-effort accommodations

    Adds a new accommodation instruction 'best_effort' that allows the
    deployment to proceed even if the number of available compute nodes
    is less than what was requested.

    For example, this will create a single pair of agents on separate
    compute nodes if possible, but will fall back to creating them on
    the same compute node if only one is available:

    accommodation: pair, single_room, best_effort, compute_nodes: 2

    Change-Id: I1986f550ac0dc934bfa5db84b40a6419d6608c7e
    Closes-Bug: 1807630