Add golangci-lint support

[marctc]

Added golanci-lint support to the pipeline. This fixes #18.
The config is taken from the Agent repo.

I fixed some lint issues but I keept the following:

vsphere/client.go:45:2: `closeGate` is unused (structcheck)
	closeGate sync.Once
	^
vsphere/endpoint.go:41:2: `pKey` is unused (structcheck)
	pKey             string
	^
vsphere/endpoint.go:42:2: `parentTag` is unused (structcheck)
	parentTag        string
	^
vsphere/endpoint.go:51:2: `parent` is unused (structcheck)
	parent           string
	^
vsphere/endpoint.go:53:2: `lastColl` is unused (structcheck)
	lastColl         time.Time
	^
vsphere/endpoint.go:68:80: newEndpoint - result 1 (error) is always nil (unparam)
func newEndpoint(cfg *vSphereConfig, url *url.URL, log log.Logger) (*endpoint, error) {
                                                                               ^
vsphere/vsphere.go:33:2: `cancel` is unused (structcheck)
	cancel context.CancelFunc

@rlankfo maybe you want to look at them as I don't have much context.

[rlankfo]

hey @marctc, thanks for putting this together. This looks good to me 👍

I'll go ahead and merge even with some of the existing lint errors but will plan to get this cleaned up shortly. Some of the unused stuff will start getting used once we are applying hierarchy labels on our metrics which should be coming soon.