Handle device features

[olofhagsand]

Devices may have enabled features in their YANG which the controller does not support.
This may lead to errors in the handling of a device YANG, in if-feature statements, where the controller assumes all such statements are false.
Devices announce features in their yang-lib state.
However, the controller at this time does not query yang-lib, only netconf monitoring state (which is translated to yang-lib module-set).