webadmin: set default plugin for provider

set default value of external network provider to 'Open vSwitch' set
'RabbitMQ' as default broker type of Agent config

Bug-Url: https://bugzilla.redhat.com/1118253

Change-Id: I52abd0ea1a6a923374e6ab094ff2a81f145f443a
Signed-off-by: Eliraz Levi <elevi@redhat.com>