Allow over-ride of non-module capabilities

We have encountered a device that does not advertise either :writable-running or :candidate capabilities even though it support writable-running. It would be useful to be able to specify the capability to use in the node configuration using something similar to yang-additional-capabilities to add or over ride the capabilities announced by the device. This is a situation where manual netconf can work around the bad hello capabilities exchange but the ODL netconf adaptor can not.

Perhaps something like:

<netconf-additional-capability>
<capability xmlns="urn:opendaylight:netconf-node-topology">
urn:ietf:params:netconf:capability:writable-running:1.0
</capability>
</netconf-additional-capability>