Enable mTLS in the client-side transport code

Acceptance criteria:

• mender.conf must respect the HttpsClient fields. They already exist in the code, but are unused at the moment.
  • Certificate and Key specify the public certificate, and the private key, respectively.
  • If either, but not both, of the above fields are set, print error, and proceed as if neither is set (this must have a test).
  • SkipVerify should be removed. It is not possible to skip verification of client certificates, since this is the server's decision, not the client's.
• When both fields above are set, enable mTLS in the client side transport code.
• Unit tests which test that the client certificate is used for all connections to the server. Use mock server from MEN-3116.

Merge to:

feature branch