This new diskimage-builder element installs octavia-lib from Git for
source install type image builds rather than from released versions.
To mention some advantages:
1. allow custom octavia-lib versions (useful for development)
2. test unreleased octavia-lib changes in CI
3. install latest changes from master and stable branches
(aligns with approach taken with amphora-agent)
Branch checkout to stable/* from master for octavia-lib DIB element will
be automated on branch create. See https://review.opendev.org/#/c/745877
Change-Id: I6d87b6bd25c536b2bed1994427cd933bdcc091d6
dcda721320