Make sure https-related packages are installed by debootstrap (Closes: #855035).

Depending on installation images (e.g. netinst), one might have
mirror/protocol set to https, but PROTOCOL set to cdrom. This means
debootstrap isn't called with an https:// parameter and doesn't know
it has to install two extra packages.

Let's look at mirror/protocol and add the apt-transport-https and
ca-certificates packages when it's set to https.