Validate --lines with `openstack console log show`

[xiexs]

The behaviors seem inconsistent for the "openstack console log --lines show", while the line number is specified with different negative numbers.
For instance:
  1) $ openstack console log show --lines -10 test_server
Invalid input for field/attribute length. Value: -9. -9 is less than the minimum of -1 (HTTP 400) (Request-ID: req-25afc42f-0fd1-45c6-b7c4-65236a5dc7a3)
        $echo $?
          1

  2) $ openstack console log show --lines -1 test_server
      $ echo $?
          0

  3) $ openstack console log show --lines -2 test_server
        >>output the all console log<<
      $ echo $?
          0

And the reason for this issue is because there is no validating for the --lines parameter.

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.openstack.org/240804

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.openstack.org/240804
Committed: https://git.openstack.org/cgit/openstack/python-openstackclient/commit/?id=ea63553925bce0432958ae4a15536f7f83b1f28d
Submitter: Jenkins
Branch: master

commit ea63553925bce0432958ae4a15536f7f83b1f28d
Author: xiexs <email address hidden>
Date: Mon Nov 9 04:22:00 2015 -0500

    Fix the bug of "openstack console log show"

    The behaviors are inconsistent while different
    negative line numbers specified.

    Change-Id: I2573f3e789f5603c896758971830ffc0b94c5e2b
    Closes-Bug: #1512263