unconfigured clients look in the [clients] section for defaults

This change enables clients that have no specific settings in
the [clients_xxx] sections of the configuration file to get defaults
from the [clients] section.

Change-Id: I071bd77a2e1f0ad366b80c095917a8debc5cef2b
Closes-Bug: 1379958
Make resource check messages more consistent

This adds a method to be used in handle_check calls to make error
messages more consistent, as well as making sure that all unexpected
messages get displayed in the status_reason field, as opposed to
stopping on the first error.

Change-Id: Id855886ae42eabd78dafc6514eca9eebf905f291
Implements: blueprint stack-check
Import sqlalchemy module instead of classes

As required by H302, import the module.
Addressing comment in https://review.openstack.org/#/c/127535/

Change-Id: If7ef3a9e403ee4fa59dc9caf020751debde2d065
Merge "unconfigured clients look in the [clients] section for defaults"
Merge "Make resource check messages more consistent"
Merge "Import sqlalchemy module instead of classes"