Support to pass Client CERT/Key and CA CERT

[jlestrada]

Please feel free to correct me where I might be wrong in this feature request. I am naive to how certificates work. Seems to be close but not exact to #26

Would like to see support to pass along CA Cert as well as Client Cert/Key. I am able to run a basic curl with the three files specified but getting TLS handshake failure with reg usage. Possibly need to create a PR if want it done but want to make sure I am not misusing before hand. Thanks!

$ ls -l
total 32
-rw-------  1 joseestrada  staff  2026 May  2 09:26 ca.crt
-rw-------  1 joseestrada  staff  4449 May  2 09:26 client.cert
-rw-------  1 joseestrada  staff  3446 May  2 09:26 client.key

$ curl -I "https://<registry>/v2/org/tags/list" -X GET --cacert ca.crt --cert client.cert --key client.key
HTTP/1.1 200 OK
Content-Type: application/json; charset=utf-8
Docker-Distribution-Api-Version: registry/2.0
X-Content-Type-Options: nosniff
Date: Wed, 12 Jun 2019 19:20:50 GMT
Transfer-Encoding: chunked
Strict-Transport-Security: max-age=7884000; preload

$ reg ls <registry>/org
INFO[0000] domain: <registry>/org
INFO[0000] server address: <registry>/orgs
Get https://<registry>/org/v2/: remote error: tls: handshake failure

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[kmeekva]

I agree with this suggestion -- we cannot use reg because our registry requires client side PKI certificate.

If I get a chance I may try to put together a PR, but want to be sure someone will consider including it before I do.