Make 'rb-site upgrade' replace the example crontab.

Because we changed search backends from PyLucene to Haystack, users need to
change their crontabs to use the new indexing commands. This change makes it so
that we replace the example crontab in the site directory with the templated
version on upgrade.

Some changes had to be made to template processing to avoid making assumptions
about some of the data, since upgrade doesn't ask any questions about domain
name, site root, etc. These aren't required for the cron template, so it all
works OK.

Testing done:
Created a 1.7.x site directory and saw that the sample crontab listed our old
PyLucene-based indexing commands. Ran 'rb-site upgrade' with this code and saw
it replaced with the new Haystack management commands.

Fixes bug 3435.

Reviewed at https://reviews.reviewboard.org/r/6096/