Remove duplicated code and a bit of dead code.

The xivo-yealink-v70 plugins references only the models for which there
is no newer firmware available, i.e. only the T32G, T38G and VP530P.
This is similar to the xivo-yealink-v72 plugin.

The xivo-yealink-v72 plugins references only the models for which there
is no newer firmware available, i.e. only the T19P and T21P. This is
similar to the xivo-yealink-v73 plugin.

The xivo-yealink-25.73.0.20 plugin is replaced by the xivo-yealink-v73
plugin.

The xivo-yealink-2015-03 plugin is replaced by
* the xivo-yealink-v80 (for T4X)
* the xivo-yealink-v73 (for most T2X and the W52P)
* the xivo-yealink-72.0 (for T19P and T21P)
* the xivo-yealink-70.0 (for T3X)

Forgot to put version in description.

As much as possible.

Since they have a few difference and it might be the easiest way to
manage the templates.

Move some config from model.tpl to base.tpl, so there is less place to
look up when searching for config. Make writing custom templates bit
easier. And previously the styles were mixed for no particular reason.

Make sure that SIP accounts are deconfigured from the phone when they no
longer appears in the xivo-provd config.

Close pull request https://github.com/xivo-pbx/xivo-provd-plugins/pull/10.

Remove the "sip_servers" block. This also fix a bug, i.e. in previous
version, if you had a backup registrar/proxy configured on your XiVO,
and then removed it, the phone would still hold the reference to the
backup registrar/proxy.

Set "account.X.nat.udp_update_enable" to 0, so that by default the phone
stops sending an empty UDP packet to the registrar every 30 seconds.

Set "account.X.fallback.redundancy_type" to 1, i.e. to "successive
registration", which is how XiVO HA works.

Set "features.caller_name_type_on_dialing" to 1, so that the phone
doesn't display the SIP username on the phone when dialing (which is
just random gibberish).

Set "sip.notify_reboot_enable" to 0, so to not force the phone to always
reboot when changing a setting.

Work in progress.

New password is xivo(9468) and impact plugins 4.0.4/5.3.0.
The default password have been changed to remove warning message on the phone.

The 1.3.5p firmware is necessary for the new Silicon Labs chip.
Otherwise, the firmware is the same as 1.3.5.

Now that all xivo-snom plugins support the same domain-less syntax for
function keys.

To VVX400/VVX410.

And move the other common.cfg from common into 4.0.4.

If you don't install the bootblock, then you'll brick your VVX1500, as
documented in http://supportdocs.polycom.com/PolycomService/support/global/documents/support/technical/products/voice/upgrading_vv1500_5_2_0_tb695.pdf

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