Run horizon-manage.py script as {{ system_user }}

Currently, we seem to always have 1/3 horizon nodes where
/var/lib/horizon/.secret_key_store is owned by root which breaks
horizon logins.  This appears to be a result of running
horizon-manage.py syncdb as root (which only happens on 1/3 nodes).
This change moves all horizon-manage.py commands to run as
{{ system_user }}, which requires us to also pre-create some dirs
owned by {{ system_user }} by adding them to container_directories so
that the horizon-manage.py commands will all complete successfully.

Lastly, we remove the creation of /etc/horizon and the chown of files
in {{ install_lib_dir }}/static in heat_common role since those have
been added to container_directories.

Closes issue #650