This was inspired by a similar change
I78a914f71cef687f09fcfee0f3f498b79d810f5d. In the bootstrap-bridge
production, we are calling create-venv every time we fire off a set of
production jobs. While it's good to keep the venv updated, it doesn't
need to happen hourly.
This writes the requirements to a file, and only installs it if the
template updates (i.e. if the venv is fresh, the dependencies updated
or we updated the daily timestamp).
Change-Id: I7a70b73fb907b923f47a2a0de72e21649c15e05f
4fa8d2dd39