Document API changes and new keystore usage

Oxygen brings changes in Netconf connection authentication as well as in device connect API. Basically we need to document these 2 models:

• New: https://git.opendaylight.org/gerrit/gitweb?p=netconf.git;a=blob;f=netconf/sal-netconf-connector/src/main/yang/netconf-keystore.yang
• Changed: https://git.opendaylight.org/gerrit/gitweb?p=netconf.git;a=blob;f=netconf/sal-netconf-connector/src/main/yang/netconf-node-topology.yang