[Docs] Rework 'Install Fuel Plugins' section in User Guide

[Irina Povolotskaya]

The following changes should be applied for the http://docs.mirantis.com/openstack/fuel/fuel-6.0/user-guide.html#install-fuel-plug-ins section:

1) plug-in -> plugin
2) add more intro information about certification
3) add link to Fuel Plugins wiki page

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.openstack.org/170824

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.openstack.org/170824
Committed: https://git.openstack.org/cgit/stackforge/fuel-docs/commit/?id=12edd1e89e5ee199371e399653b89391b29103b9
Submitter: Jenkins
Branch: master

commit 12edd1e89e5ee199371e399653b89391b29103b9
Author: Irina Povolotskaya <email address hidden>
Date: Mon Apr 6 14:57:07 2015 +0300

    Edit 'Install Fuel Plugins' section in User Guide

    This section should be reworked to provide:

    * up-to-date content

    * actual links

    Change-Id: Ic4ae5f7463130f22a8d6a50702f8d4103e371a41
    Closes-Bug: 1440711

[Irina Povolotskaya (ipovolotskaya)]

https://docs.mirantis.com/openstack/fuel/fuel-6.0/user-guide.html#install-fuel-plugins - fixed in 6.0 as well

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.openstack.org/177096
Committed: https://git.openstack.org/cgit/stackforge/fuel-docs/commit/?id=e0db1e11cf7b51fedc7e48262033672256f4cc9c
Submitter: Jenkins
Branch: master

commit e0db1e11cf7b51fedc7e48262033672256f4cc9c
Author: Irina Povolotskaya <email address hidden>
Date: Fri Apr 24 09:42:11 2015 +0300

    Adds link to Fuel Plugins wiki page into User Guide

    Currently, Fuel Plugin Guide was removed so we need
    to refer to one source of information that is
    constantly updates - Fuel Plugins wiki page.
    Changes will be made to Install Fuel Plugins section
    of User Guide.

    Change-Id: I7cb319940ba1704300583477c009930fdd23fc46
    Related-Bug: 1440711