This drops support for VVX1500, which requires a special procedure to
upgrade to 5.3.0 and is not officially supported.

FIX yealink 2015-03: zip files instead of rar files

xivo-yealink-2015-03: Update firmware to latest version x.73.0.50

Replaced by the newer xivo-snom-8.7.5.17 plugin.

This drops support for the Snom 360, which is not supported by the
latest firmware.

Some of these models are not officially supported by XiVO, but still
place them in this plugin.

So that the display behaviour of the D7 expansion module is the same as
previously.

Also homogenize a little some parameters between the differents models.
There is still work to be done there.

Note that regarding the auto_provision.custom.protect parameter, not all
models supports it; all phones in version earlier than V73 do not. This
includes:

* T32G
* T38G
* T19P
* T21P

Deprecated by the 3.3.1-SP4 plugin.

Use the latest SP4 firmware (i.e. HF1/HF2). Note that this plugin has
never been pushed to stable yet.

Now that the 6800 series are supported by the 4.0.0 plugin, it's simpler
if the 3.3.1 plugin, which has never been pushed to stable, doesn't include
them.