Set "User-agent" in request header when making requests from indexer to ns_server

Indexer has multiple components that query various ns_server endpoints. When such a query from indexer happens, ns_server would log it in ns_server.http_access_internal.log. Below is the format:

127.0.0.1 - @index-cbauth [04/May/2020:05:39:18 -0700] "GET /pools/default/buckets/other-3/collections HTTP/1.1" 200 98 - "Go-http-client/1.1" 1

The format of this message is:

Node IP - User_of_the_request [Timestamp] "Request path" Response_status Size_of_response Referer User-agent Response_time

Currently, the User_agent value mentions "Go-http-client/1.1" which does not convey any valuable information. Instead, each request from indexer can set the appropriate name of the client as User_agent i.e. RetrievePlanFromCluster can set User-agent as "RetrievePlanFromCluster" and it would get logged as below:

127.0.0.1 - @index-cbauth [04/May/2020:05:39:18 -0700] "GET /pools/default/buckets/other-3/collections HTTP/1.1" 200 98 - "RetrievePlanFromCluster" 1

This would help us to associate a request with it's client facilitating easy debugging