Allow changing any horizon settings from the plugin config files

Sometimes we want our pluggable dashboard to change more than just
the selected handful of options in the HORIZON_CONFIG. This patch
lets the configuration files for the plugins add and modify any
horizon settings.

Change-Id: I25aec577733631d1e2057bde0b12f1e50c56b05b
Closes-bug: #1302387
Fixes wrong logic for clean method in SetInstanceDetailsAction

If the 'Instance Boot Source' is 'Boot from image (creates a new
volume)', the 'Device Size' and 'Device Name' fields will never be
checked.

Change-Id: Ic34de83a46716f6489d017275f9b64ce0c951858
Fixes-bug: 1313473
Hide disk config option if the extension is not supported

Change-Id: I3a30d055ca2f419f5427e62e8b70f66153b14927
Closes-Bug: #1306302
Fixing Neutron Subnet Details help text

Removing the '&lt;' and '&gt;' tags which were wrongly displayed in
the help text of the Subnet Details dialog for Neutron Networks /
Subnet create.

Closes-Bug: #1311820
Change-Id: I9e730a2d90a6174634c3233230e48e63549e1b9c
Added tabindex value to links in the sidebar.

The tabindexes are reseted for each panel, beginning with 1. Because
of the accordion nature of the interface only the links of one of them
will be visible at any given time (the elementts with display:none
will not be focusable).

Change-Id: I3556a3f5b3b2db13c77ed6434b3f54ca7a530dc4
Closes-Bug: #1308442
Added the field label to the field arguments.

Initially, the field key was used as the label. In the return dictionary
has the label, so I use that value and fall back to the key if it does
not exist.

Change-Id: I39b4a7f25d8947276d3bdc3e12e728f84696a203
Co-Authored-By: Yves-Gwenaël Bourhis <yves-gwenael.bourhis@cloudwatt.com>
Merge "Added the field label to the field arguments."
Merge "Fixing Neutron Subnet Details help text"
Merge "Added tabindex value to links in the sidebar."
Merge "Hide disk config option if the extension is not supported"
Merge "Fixes wrong logic for clean method in SetInstanceDetailsAction"
Merge "Allow changing any horizon settings from the plugin config files"