Fix docker volumes binds issue

Without the fix, the volumes will not be attached after the container
started.

Closes-Bug: #1322205

Change-Id: Ie8eff402e8714f3c5d478dbc9fc02b43a377dccd
Support loading constraints as stevedore extensions

This still maintains support for the existing mechanism
but moves the in tree constraints to using stevedore.

part of blueprint stevedore-plugins
Change-Id: I86907e6518281083a508f840269d79dfa3875e9a
Base class for client plugins

Subclasses must implement the _create method which is only called the
first time client() is called. A reference to the Clients object is
stored to allow client plugins to access other plugins.

auth_token and url_for are utility methods which can be called by
subclasses or elsewhere in heat.

_get_client_option is a private utility method for subclasses.

Change-Id: I5f890a3632b65a8cb534e0cf4dac858166bc168a
Merge "Fix docker volumes binds issue"
Merge "Support loading constraints as stevedore extensions"
Merge "Base class for client plugins"