ostree_sysroot_init_osname: also create /var/log
ostree_sysroot_init_osname: also create /var/log

/var/log is another one of those core directories that should be made
available and properly labeled during early boot before tmpfiles.d
starts up.

Related: https://bugzilla.redhat.com/show_bug.cgi?id=1265295

Closes: #513
Approved by: cgwalters

GNOME/ostree,GNOME/ostree,GNOME/ostree,GNOME/ostree,GNOME/ostree