Add a new "full_center" layout

[himdel]

Add a new "full_center" layout - similar to center_div, but fully customizable - you just get the menus

the idea is for specific controllers to

    helper do
      def layout_full_center
        "layouts/my_custom_layout"
      end
    end

#3963: needed so that v2v doesn have extra margins around it

[himdel]

Cc @mzazrivec @martinpovolny

[miq-bot]

Checked commits https://github.com/himdel/manageiq-ui-classic/compare/184d0b5071665ff78b2f7c48f185e5e0eedc8744~...8476d59484820a8e3d1a785a36bf8d0af349ab5f with ruby 2.3.3, rubocop 0.52.1, haml-lint 0.20.0, and yamllint 1.10.0
3 files checked, 1 offense detected

**

• 💣 💥 🔥 🚒 - Linter/Haml - Linter::Haml STDERR:

warning: parser/current is loading parser/ruby23, which recognizes
warning: 2.3.6-compliant syntax, but you are running 2.3.3.
warning: please see https://github.com/whitequark/parser#compatibility-with-ruby-mri.

[simaishi]

Gaprindashvili backport details:

$ git log -1
commit 136e27aeefdda594ff629e63dab8dbf2d0c0e7ff
Author: Martin Povolny <[email protected]>
Date:   Tue May 22 19:23:00 2018 +0200

    Merge pull request #3970 from himdel/v2v-layout
    
    Add a new "full_center" layout
    (cherry picked from commit 8d884ce5c3f10635ce96eeb518b44e089063cc9e)