This PR adds the 'choose' query parameter to the '/v1/service/<service>' endpoint.

The value of 'choose' is in the form '<number>|<key>', number is the number
of desired services and key is a value unique but consistent to the requester
(e.g. allocID).

Folks aren't really expected to use this API directly, but rather through consul-template
which will soon be getting a new helper function making use of this query parameter.

Example,

curl 'localhost:4646/v1/service/redis?choose=2|abc123'

Note: consul-templte v0.29.1 includes the necessary nomadServices functionality.