Cherry-pick trebuchet pull #15

https://github.com/trebuchet-deploy/trebuchet/pull/15 fixes the initialization
of repositories with submodules on tin by setting the checkout-submodules
flag. This means we'll have one less thing to work around during set up of a
new project.

Change-Id: Ibe59cce091563f640744705e482b37aa24e149ed
Merge "Cherry-pick trebuchet pull #15" into production