Fix expiration command in expire_old_reviews.

* jeepyb/cmd/expire_old_reviews.py(expire_patch_set): The command
format string passed to Gerrit's SSH CLI was missing a space between
the options. Regression introduced when or before this script was
pulled out of opestack-ci-puppet, but fixed now.

Change-Id: I96ff9527d635555e1ee2c4110544c266a745f42f
Reviewed-on: https://review.openstack.org/18541
Reviewed-by: Clark Boylan <clark.boylan@gmail.com>
Approved: Jeremy Stanley <fungi@yuggoth.org>
Reviewed-by: Jeremy Stanley <fungi@yuggoth.org>
Tested-by: Jenkins