Setup a usual jenkins slave

Support the following variables:

install_jenkins_utilities: (true) install usefull utilities including qemu/debootstrap/fakeroot
install_osmocom_build_deps: (true) install all osmocom runtime and build time dependencies
generic_slave: (true) contains tasks used by the most osmocom jenkins slaves
osmocom_jenkins_slave_fstrim: (false) calls fstrim periodical
ttcn3_slave: (true) install titan ttcn3 compiler and prepares the docker-playground