Fix path for cron-based run.
Fix path for cron-based run.

pull-repo.sh is run from /usr/local/mirror_scripts/run-mirror.sh
which is run from cron. cron is not setting /usr/local/bin in the
path - so pip isn't found.

Change-Id: I31f8b5837204d38d0371db59a1a03b8bd49b508a

open-switch/infra_puppet-jenkins,open-switch/infra_puppet-jenkins