Ensure api_paste_conf is an absolute path

If api_paste_conf is specified as a relative path and the directory from
where the nova-api service is started contains a file with the same
name, the wsgi loader won't try to find the absolute path to that file
which is required when not passing the base directory to the
paste.deploy library.

This change ensures the config_path is an absolute path so it can be
found by the paste.deploy library.

Closes-Bug: #1259183
Change-Id: I0eb6ef1d69d302634b8c842449f997ec10c13933
Merge "Ensure api_paste_conf is an absolute path"