add support to deploy from local charms

original lp bug - https://bugs.launchpad.net/stsstack-bundles/+bug/1893800

Add scripts to deploy using charms 2 years old (e.g. 18.05), these scripts should do something like this:

1) ./clone-all.sh --branch stable/18.05 --target /home/ubuntu/charms/

Internally the script would do something like:

cd $TARGET_DIR
for CHARM in $LIST_OF_CHARMS; do
git clone https://opendev.org/openstack/${CHARM}.git
pushd $CHARM
git checkout $BRANCH
popd
done

2) ./generate-bundle --charms-dir /home/ubuntu/charms/

When --charms-dir is passed a new overlay is generated that contains lines like these:

keystone:
charm: /home/ubuntu/charms/keystone