Allow configuring the depth of the token bucket in the rate limit api

[luke-clifton]

We would like to be able to set rate limits that support bursty traffic. The current rate limit is supposedly based on a token bucket, however we can't configure the depth of the bucket independently of the rate.

As it currently stands, the buckets depths is equal to it's rate. So if we want to allow people to make bursty requests to Vault, we need to set a really high rate, which defeats the purpose.