discovery: make gimbal client request rate limit configurable

[alexbrand]

Fixes #150

This PR makes the client request rate limits against the Gimbal kubernetes API server configurable via command-line flags on both the kubernetes and openstack discoverers.

New flags:

flag	default	description
--gimbal-client-qps	5	The maximum queries per second (QPS) that can be performed on the Gimbal Kubernetes API server
--gimbal-client-burst	10	The maximum number of queries that can be performed on the Gimbal Kubernetes API server during a burst

I decided to go with the client-go defaults, which means that the user will most likely always want to set these flags when deploying Gimbal.

With that said, it might be worth setting defaults that are higher, but I am not sure what those would be. I am also interested in feedback on the flags themselves and the description/documentation.

[stevesloka]

I think having them configurable is good as a first step and some of the performance testing will help to come up with values we can recommend.

The only thing we could possibly document better is when to tweak one vs the other. Like what is burst and when does it come in to play? Otherwise LGTM.

[alexbrand]

Sounds good! I'll add some more documentation around QPS and burst to the discoverer docs.

[alexbrand]

Added documentation on qps and burst. I hope I am on the right track, but would love some feedback.