add usage to configure node web console in doc install-guide.rst

[baiyuan]

There is no usage to configure node web console in doc install-guide.rst; For some new users, they do not know how to configure web console for ironic correctly, so I add following in doc install-guide.rst: install shellinabox in ironic conductor and configure web console for node.

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.openstack.org/176642

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.openstack.org/176642
Committed: https://git.openstack.org/cgit/openstack/ironic/commit/?id=ba2d4efd7a1717dcf222cca6aa1829495c8be78a
Submitter: Jenkins
Branch: master

commit ba2d4efd7a1717dcf222cca6aa1829495c8be78a
Author: baiyuan <email address hidden>
Date: Thu Apr 23 02:45:05 2015 -0400

    Add documentation for getting a node's console

    This commit adds documentation for configuring and
    getting information about a node's console.

    Change-Id: I2cde0b18a67e2b3a501f2a51904735bc79def440
    Closes-Bug: 1447465