Add --head-only and --package-name options.

Adds a new --head-only option which can be used to make Delorean
build only the most recent Git commit for each project.

Also adds a new --package-name option which can be used to
build only a specific package.

As part of these changes the getinfo function was refactored to
return a project "to process" list rather than passing one into
the function. We also moved some of the logic out of getinfo to
make things cleaner.

Change-Id: I0cd77058af6ddca709956675f23b501c2997fc83
Email changes
Populate the build log with something if its empty

People get pointed at it in emails, and its confusing if it contains nothing.
Install pbr before creating the sdist.

Updates the build_rpm.sh script so that we pre-install
python-pbr. Many OpenStack projects use pbr as
part of setuptools and pre-installing it as an RPM
prevents it from being downloaded via pip.

Change-Id: I3edf27ede9bbcb38ecfd7d3121c404faf37d6c14