tools/rbd/action: align column headers left

tools/rbd/action: align column headers left

Even if the column content is right-aligned, we should align the column
header left.  That way intead of

  SNAPID NAME  SIZE TIMESTAMP                NAMESPACE
       4 snap 1 GiB Mon Jun  4 16:27:17 2018 user

which has makes it hard to visually parse what that 1 belongs to (NAME or
SIZE?), we get

  SNAPID NAME SIZE  TIMESTAMP                NAMESPACE
       4 snap 1 GiB Mon Jun  4 16:27:17 2018 user

Signed-off-by: Sage Weil <sage@redhat.com>