Fix failing Elasticsearch test

Use the local instance of `ReplaySettings` rather than `Settings` which is
initialised as an empty struct and then populated by flag. Since we're not
calling flag, this remains empty, and `esp.Init()` is passed an empty
string.

For the same reason the call to `settings.Parse()` is actually redundant.
`Parse()` could be changed to use `r.ElastiSearchURI`, but it results in the
plugin begin setup/registered twice, because it doesn't give us access to
the host/port/index properties.