Create localhost MySQL accounts for keystone,glance,nova.

Document that you need to create an @'localhost' account as well as
an @'%' account in MySQL for keystone, nova, and glance.

Cherry-picked from https://review.openstack.org/16828

(This is an old commit to master that never got pushed back into
folsom)

Change-Id: I4fcbf03a6cd443bbe94a5f41f91bbccd08017efd