Make pull request closer less OpenStack specific.

* jeepyb/cmd/close_pull_requests.py: The pull request closer was
thanking people for contributing to OpenStack. This script is also used
to close stackforge pull requests. Make the message less OpenStack
specific and instead thank the pull requester for contributing to the
project they pull requested against.

Change-Id: I6bc29401ab72dc452c3c26fc11ffb6314c6bc7bd
Reviewed-on: https://review.openstack.org/26771
Reviewed-by: Elizabeth Krumbach <lyz@princessleia.com>
Reviewed-by: James E. Blair <corvus@inaugust.com>
Approved: Jeremy Stanley <fungi@yuggoth.org>
Reviewed-by: Jeremy Stanley <fungi@yuggoth.org>
Tested-by: Jenkins