webtry: Refactor instance name constant.

Make it use VM_NAME_BASE constant like we are doing now for monitoring
and perfserver.

BUG=None
R=rmistry@google.com

Review URL: https://codereview.chromium.org/737243003
prober: The server is Graphite not Carbon.

Carbon is just the protocol used to communicate with InfluxDB.

R=jcgregorio@google.com

Review URL: https://codereview.chromium.org/744863002