Make getmeza.sh configurable

[freephile]

• Make the project git repo a configuration value #22
• Make the project branch a configuration value #23
• Make the branch depth a configuration value #24 - this is for the mediawiki role, not getmeza.sh

This issue is one instance of issue #1

getmeza.sh currently looks like this:

# if /opt/meza doesn't exist, clone into and use master branch (which is the
# default, but should we make this configurable?)
if [ ! -d "${INSTALL_DIR}/meza" ]; then
	git clone https://github.com/enterprisemediawiki/meza.git ${INSTALL_DIR}/meza --branch master
fi

1. We need to make the repo configurable (so forking is easy)
2. We need to make the branch configurable (so you don't need an immediate extra step to work on a new release)
3. We need to add a 'depth' parameter (because the full .git object database for MediaWiki is over 2GB!!)