repo:voltha-system-tests: docker/Dockerfile installs a hardcoded version of voltctl

Update logic to remove hardcoded voltctl download and install.

Update logic to retrieve voltctl based on release:

• branch=master, download binary from release tag latest.
• At release time: update logic to checkout version appropriate for release.

Tool install logic should even be refactored to another layer.  Other scripts perform the same task – installVoltctl.groovy contains hardcoded versions to checkout by branch.

A single tool installer should be setup that is aware of branches and release tags.

Checkout and install binary to a local directory.  All jenkins jobs and scripts should use this central installer.