Allow expire_old_reviews to be used with suexec

The suexec command permits executing any other command as any other
registered user account, using the Gerrit ssh key key.
(http://gerrit-review.googlesource.com/Documentation/cmd-suexec.html)

It may be convenient to use this command with expire_old_reviews, to
avoid having to maintain an extra ssh key on the server for admin tasks.

This change adds an optional argument to specify the user to expire
reviews as.

Example:

expire-old-reviews 'Gerrit Code Review' \
  /var/gerrit/review/etc/ssh_host_rsa_key --as_user jenkins

Change-Id: I8003d74b5bc7f74d853a21f97fb5aef1b7a31df7