Changes

Jenkinsfile: remove .git mount in stages that don't use it
The .git mount is only needed for the DCO check, and for building the
binaries if `DOCKER_GITCOMMIT` is not set.
Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

Jenkinsfile: inline janky steps, and move validate to separate stage
Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

Jenkinsfile: inline experimental, power, z steps, and split Unit test
Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

Jenkinsfile: move docker-py to separate stage
Signed-off-by: Sebastiaan van Stijn <github@gone.nl>