Changes

testenv: add --install-package

Add an option to quickly install packages in the container. This is
useful when making a new SUT run with osmo-ttcn3-hacks and figuring out
which packages need to be installed to make the build pass (without
rebuilding the whole container for each missing dependency). It can also
be used to quickly install additional debugging tools (strace, valgrind,
etc.). A cache for the deb files is already getting mounted inside the
container.

Change-Id: Ie54817e6c0334a224a612521beb378537c10d39d