ManagementServerImpl: Fix NPE, PluggableService should not return null list

Signed-off-by: Rohit Yadav <bhaisaab@apache.org>
ApiServer: Ask pluggable services to return list of apis to ApiServer

Signed-off-by: Rohit Yadav <bhaisaab@apache.org>
ApiDiscovery: Discovery apis provided by PluggableServices as well

Signed-off-by: Rohit Yadav <bhaisaab@apache.org>
cli: Fix attribute error and fix helpdoc

Signed-off-by: Rohit Yadav <bhaisaab@apache.org>