Makefile: `enable_stbt_camera=no` does what you'd expect

Previously only an empty value for `enable_stbt_camera` would disable
it.
Makefile: Cleanup: `stbt-camera` isn't a build artifact
Makefile: Cleanup spacing of ":" in rules

For consistency, and so that I can find the "install" target (as opposed
to other targets like "install-core" or "install" commands in recipes)
by searching for "install:".
Remove extra/fedora/Vagrant*

(These were to provide a host linux system for running docker in, to
build stb-tester's Fedora packages inside a docker container.)

With docker 1.3, boot2docker for OS X supports shared directories on the
host, so vagrant for a host linux system isn't necessary any more.
Anyway I have a Linux host system now.
tests/run-tests.sh: Cleanup: Rename internal variable

"test_installation" is ambiguous: Is it a verb (test the installation)
or a noun (the test installation)? If it's a verb, does it mean to test
the installation process or to test the version that's already
installed? (It's the latter.)
extra/fedora: Use the (semi-) official Fedora docker image