buildah mount should work like the mount command and list all mount points
when no options are specified.
Need buildah umount to remove mount point from the database when a mount point
is umounted.
Also remove Mounts field from the builder object. We only support a single mount
point so no reason for this field any longer.
Signed-off-by: Dan Walsh <dwalsh@redhat.com>
Closes: #124
Approved by: nalind