Add support for SignalFX

As per the [spec](https://github.com/interchainio/testnets/blob/master/docs/network-layout-spec.md), we need support for [SignalFX](https://www.signalfx.com/).

We already have the configuration parsing going at present, but we need the SignalFX agent to be deployed to the Tendermint hosts and configured, and for that we'll most likely need a new [Ansible role](https://github.com/interchainio/testnets/tree/master/tendermint/ansible).

The relevant part of the code in the `tmtestnet` tool for deploying monitoring would also need to be updated.