Enable ClusterInfoService by default

Since we enabled the disk threshold decider by default, we need to
enable the cluster info service so that disk usages and shard sizes can
be gathered also.

Adds a test that checks that we are gathering information by default.

I also talked to @bleskes about how to test both the
`InternalClusterInfoService` and `DiskThresholdDecider` at an integration
level, and he suggested mocking the responses at the transport layer. I
will work on that for a subsequent pull request after this one.