Buck: Deploy plugin artifacts to Maven Central

Switch deploying Gerrit extension and plugin API from Google storage
bucket to Maven Central.

To simplify the troubleshooting new trace option was added to
underlying python script. When environment variable VERBOSE=1
is set, then the mvn command is printed to stderr:

  VERBOSE=1 buck build api_deploy_oss
Change-Id: I9ebe44ec8aca91aa6b5514f561209e7c50377927
Merge "Buck: Deploy plugin artifacts to Maven Central" into stable-2.9