Update keystone driver config settings for liberty

[Colleen Murphy]

In Liberty, keystone will move to using stevedore to load drivers: https://blueprints.launchpad.net/keystone/+spec/stevedore

This will change the default values and allowed values for various driver settings in keystone.conf. For example, the identity/driver would be set to 'sql' instead of 'keystone.identity.backends.sql': https://github.com/openstack/keystone/blob/master/etc/keystone.conf.sample#L799-L800

We should make sure to set these params to ensure => absent if unset, so that configs use the default value (so we don't have to update again next time keystone changes its mind), and change any string validation and documentation to accept the new driver strings.

[Emilien Macchi (emilienm)]

This patch has to be done *after* Kilo release so we can actually test it, just for information.

[Richard Megginson (rmeggins)]

Can we do this now?

[Matt Fischer (mfisch)]

Fixed with 1f051ca9b7f704d7bafc1933bbc86653607d0b12