Fix restconf-nb websocket initialization failure on karaf

Fix restconf-nb websocket initialization failure on karaf

Websocket servlet is initialized using indirect dependency on
server bundle (string class name in code, dynamic-import in
manifest). This dependency require to be set explicitly. Then
required bundle will be installed with odl-restconf-nb feature
and missing artifacts will be available for restcong-nb
class loader.

JIRA: NETCONF-953
Change-Id: I82e9133af1aeb98518f97a14a3574b82573f92dc
Signed-off-by: Ruslan Kashapov <ruslan.kashapov@pantheon.tech>