added openssh-server

required for vagrant ssh
added centos 7 target and change lxc config for compatibility

lxc.cap.drop = setpcap causes really slow boot rendering the container practically useless
lxc.aa_profile = unconfined is needed to run centos 7 on systems with apparmor (unclear if there is an alt/better solution)
lxc.autodev = 1 apparently helps systemd ...
lxc.kmsg = 0 ensures that systemd-syslogd doesn't eat up all your CPU cycles