Add portieris version to user agent for registry requests.

[brackendawson]

There's kind of a TODO here:

portieris/pkg/verifier/simple/imagePolicy.go

Line 48 in 1b2cfa1

DockerRegistryUserAgent: "portieris", // add version?

[brackendawson]

There are also two user agents in this package:

portieris/pkg/notary/notary.go

Line 93 in 0640ab2

"User-Agent": []string{"portieris-client"},

[brackendawson]

It would be good to also set the user agent on this config struct for the kube API:

portieris/helpers/kube/kube.go

Line 31 in 0640ab2

var config *rest.Config

There are also several other clients in the helpers directory that would benefit from one, such as oauth.