tokenAuth pingClient should not assume the "v2" url is at the root.

[reasonerjt]

If I host my notary-server under a sub path:
x.x.x.x/notary
I can use the command:

notary -s https://x.x.x.x/notary list ......

to list the targets, however when the "auth" is configured at the notary-server, the command will not challenge me for credentials.

Checked the code:
https://github.com/docker/notary/blob/master/cmd/notary/tuf.go#L892

the "/v2/" url is hard coded to the root, but in my case the notary client should "ping" the url x.x.x.x/notary/v2 instead of x.x.x.x/v2

a quick fix:

var p string
if strings.HasSuffix(endpoint.Path, "/") {
     p = endpoint.Path + "v2/"
} else {
     p = endpoint.Path + "/v2/"
}
subPath, err := url.Parse(p)

can work around the problem.

[ecordell]

👍 This is important for potential non-container-registry Notary users.

IMO, we should just have a separately configurable auth URL which is perhaps by default tuf_url+ /v2

[reasonerjt]

@ecordell
so that's what my fix in OP is doing, I'll open a PR if you think it looks OK.