Merge lp://staging/~sinzui/juju-release-tools/build-source-packages into lp://staging/juju-release-tools

Add source package creation to build_packages.py

This branch rethinks how we make source packages to reduce duplication, run unattended, and run faster. The shell scripts were tuned and inlined into the python.

1. Add JujuSeries to help find series, versions codenames, and status
2. Instead of creating a new source pacakage branch by importing the
   release tarfile for each series, the action is done once.
3. The rules to create the ubuntu version string and changelog
   message are simplified because we now trust CI to make source
   packages
4. Source packages are always made unsigned, if a gpgcmd is provided,
   the package will be signed afterwards. We don't have two ways to build
   a package now. The call to dch now uses "--force-distribution" to
   prevent a prompt to approve unknowwn series (like wily)
5. The directory tree is very regular:
   SPB: $WORKSPACE/juju-build-any-all
   SP: $WORKSPACE/juju-build-<series>-all
   build: $WORKSPACE/juju-build-<series>-all/build-area
   source: $WORKSPACE/juju-build-<series>-all/source
   ^ We care about the files in each $WORKSPACE/juju-build-<series>-all/*.*

I used this script yesterday when attempting a release of 1.24 (we aborted)

build_package.py -v source \
    --debemail "$DEBEMAIL" --debfullname "$DEBFULLNAME" \
    --gpgcmd $JUJU_HOME/cloudgpg \
    juju-core_$VERSION.tar.gz ./ LIVING $BUGS