Unified Docker client versioning

Use a single client version for all Docker API calls.

Until now, client methods were being upgraded adhoc. That
was easier to maintain in absense of tests, but with tempest
now running, the cost of upgrading these methods should be lower.

API version bumps can cause behaviorial changes. Docker itself
attempts to keep API behaviors consistent, but this also means
we might miss positive behavorial changes such as pause-on-commit
if the API requests are not upgraded.

Change-Id: Id2f6889db75138a0d3e54d8b91e7f19cf658fdad