* Support to set client config via env variable
Currently some configuration such as `kube-api-burst` and
`kube-api-qps` must be set via command line option.
Knative deployments usually do not have the command line option,
the command line option is unusual and so knative/operator also does
not support the API.
Hence This patch allows to set client config via env variable.
* only qps and burst
* Add copyright
* Handle float
* Add error handling
* consolidate k8s flags to an environment package
* add copyright
* fix comment style
* use go1.16 in workflows so downstream tests work
* Deprecate GetRESTConfig and do not remove
* update copyright date
David Simansky
Kenjiro Nakayama
Ben Moss
Dave Protasowski