In the near future, we will need to communicate with Koji using HTTPS.
This will surely bring the need for ignoring bad certificates/providing
our own self-signed ones. Thus, this commit prepares the Koji integration
by adding a way to accept a custom http transport which can be used to
customize the TLS settings.
Previously, Koji instance could be both logged-in and not logged-in.
This change disallows it: Now, the Koji instance is created by calling
koji.Login, so it must be always logged-in. This change should lead to more
robust code.
