[WIP] Default Branches: Permit repos to have a default branch

[tombettany]

https://trello.com/c/MAJSh2Bd/219-3-make-a-rc-package-build-script-or-modify-dr-so-dr-update-can-work-from-a-specific-branch

Currently, the way of determining which branch should be used, should the branch CLI option not be provided, is to use the currently checked out branch. This makes it difficult to specify a policy for a suite, or even have a different branch for each suite.

Add the beginnings of an approach to permit default branches to be specified by providing an option in the suite definitions which can be overridden by the package specific options.

@pazdera I'm looking for comments here as I'm not certain that I've gone the correct route. Also, I had some troubles deciding what the default behaviour should be so that we maintain backwards compatibility with repos which do not know about the setting.