Introduce broker-cli tool

[bmelville]

The broker-cli tool allows for debugging and managing brokers from the Google Cloud Service Broker API. It uses the existing broker-cli library already existing as part of the installer.

Introduction of this tool will require the following steps:

• [x] Refactor existing broker-cli code out of installer into its own top-level directory
• [x] Vendor broker-cli library into installer to decouple
• [x] Remove old broker-cli from installer/pkg
• [x] Introduce recent changes/additions to the library to support the tool
• [x] Introduce the tool itself

[mihnjong]

@martinmaly @maqiuyujoyce

Joyce found two copies of broker-cli code in the repo. Now, I see what Brendan was planning. Joyce, could you take it over ?

[mihnjong]

The next step is "Remove old broker-cli from installer/pkg"

[maqiuyujoyce]

broker-cli at the root and all the unused files under installer/pkg/broker-cli has been removed.