Setup CPaaS midstream repo on 3.6 branch

Acceptance criteria:

• Trigering the CPaaS build updates the dist-git repo with sha of newest upstream changes
• Each midstream in gerrit should contain a 3.6 branch based on the 3.7 branch
• Each midstream 3.6 branch should contain dist-git contents for the equivalent 3.6 branch
• upstream_sources.yaml should contain correct upstream branch name
• All references to 3.7 are updated to 3.6