install: Make DBus-related packages optional

Installing Glib seems to pull a lot of things that are not cleanly packaged, leading to a range of cryptic errors at install time. Since bisector uses GLib's DBus bindings, do not install that optional feature by default.

Also allow install_base.sh to register extra requirements for pip, to be installed by lisa-install. This gives a way of enabling an optional feature when running ./install_base.sh, without having to also add a way of specifying that for lisa-install.