Update omnibus to 5.6.10
Omnibus is now at at 5.6.10, it includes the PR https://github.com/chef/omnibus/pull/823 which is needed as part of #3020 (closed)
We have gitlab-omnibus-v5.6.3 branch, which is 5.6.3 + gitlab patches. Create gitlab-omnibus-v5.6.10 from 5.6.10 plus our patchset.