rename loggregator_endpoint to metron_endpoint

We're introducing an agent called "metron" on every VM that needs to send logs to loggregator. In preparation for this change, we're renaming the "loggregator_endpoint" spec/manifest setting to "metron_endpoint". The change has already been done in cf-release/develop, but since cloud controller templates live in their own repo, we also need to change it here.

Related story: https://www.pivotaltracker.com/story/show/74960322