run: don't pass certain flags to systemd-nspawn for systemd < 209
run: don't pass certain flags to systemd-nspawn for systemd < 209

The -q/--quiet flag and the --register flag were added to systemd-nspawn
in version 209, and some distros (like CentOS 7) ship with an older
version than that. This commit runs and parses `systemctl --version` to
determine the version number of systemd, and will only pass these flags
if the version is high enough.

joshix/acbuild,jonboulle/acbuild,dgonyeo/acbuild,appc/acbuild,mikeln/acbuild,appc/acbuild,mikeln/acbuild,jonboulle/acbuild,asbachb/acbuild,containers/build,containers/build,asbachb/acbuild,joshix/acbuild,dgonyeo/acbuild