Support DC/OS install on non-root LVM volume

We need to support installing DC/OS to a non-root LVM volume. Currently DC/OS insists on creating and populating /opt/mesosphere/ during installation. This also has implications for systemd: we symlink unit files from within /opt/mesosphere/ to /etc/systemd/system/, and the volume to which DC/OS is installed needs to be mounted before systemd starts DC/OS.