lorax: Save information about rootfs filesystem size and usage

Run df on the filesystem image after it has been created.
Output will be in program.log, eg:

Running... df /var/tmp/lorax.imgutils.wm04pg_v
Filesystem     1K-blocks    Used Available Use% Mounted on
/dev/loop0       1998672 1619508    362780  82% /var/tmp/lorax.imgutils.wm04pg_v
Return code: 0

src/pylorax/imgutils.py

@@ -477,6 +477,9 @@ def mkfsimage(fstype, rootdir, outfile, size=None, mkfsargs=None, mountargs="",
                 copytree(rootdir, mnt, preserve)
             do_grafts(graft, mnt, preserve)
 
+            # Save information about filesystem usage
+            execWithRedirect("df", [mnt])
+
     # Make absolutely sure that the data has been written
     runcmd(["sync"])