Deprecate redundant default-virtual-host of Undertow subsystem root resource

The Undertow subsystem has 2 redundant attributes that define the default host of the default server.

/subsystem=undertow contains a "default-virtual-host" attribute, that, together with the "default-server" attribute, identify the default host of the default server.

However, the /subsystem=undertow/server=* resource also contains a "default-host" attribute that, together with the default-host attribute of the /subsystem=undertow resource, identifies the default host of the default server.

This problematic for a few reasons:

1. It is confusing for users
2. Since these 2 attributes could easily specify 2 different value, it can easily result in invalid configuration