Currently the proxy expects the container name as the first element
of the path. It then adds a prefix followed by a "-" character to
the container name before using it.
To additionally support the case where no prefix is desired (so that
the path that the Zuul base job returns is the entire path used by
the proxy), remove the automatic addition of the "-" character. This
means that if no prefix is supplied, then the path is used as given.
To obtain the previous behavior, add a "-" character to the end of
the CONTAINER_PREFIX environment variable.
Change-Id: Ia480a11f78621ea0b2417f4b47878640f52db5da
7b2cba8657