netconf-client-mdsal emits useless namespace in filter

When we emit a filter we end up doing this:

<filter xmlns:ns0="urn:ietf:params:xml:ns:netconf:base:1.0" ns0:type="subtree" xmlns="urn:ietf:params:xml:ns:netconf:base:1.0">
    <top xmlns="http://example.com/schema/1.2/config">
        <users>
        </users>
    </top>
</filter>

The problem here is that we emit a namespace declaration to host the 'type' attribute, which is wrong. The interpretation here is that attributes inhering the default namespace, but that is not true: attributes do not inherit the the namespace of the element in which they are declared.

In this particular case "type" should just be a plain attribute, without any prefix:

<filter type="subtree" xmlns="urn:ietf:params:xml:ns:netconf:base:1.0">
    <top xmlns="http://example.com/schema/1.2/config">
        <users>
        </users>
    </top>
</filter>

and without the ns declaration when we already have it in an ancestor element.