change default to be on (sonic-net#13495) (sonic-net#13796)

Changing the default config knob value to be True for killing radv, due to the reasons below:

Killing RADV is to prevent sending the "cease to be advertising interface" protocol packet.
RFC 4861 says this ceasing packet as "should" instead of "must", considering that it's fatal to not do this.
In active-active scenario, host side might have difficulty distinguish if the "cease to be advertising interface" is for the last interface leaving.
6.2.5. Ceasing To Be an Advertising Interface

shutting down the system.
In such cases, the router SHOULD transmit one or more (but not more
than MAX_FINAL_RTR_ADVERTISEMENTS) final multicast Router
Advertisements on the interface with a Router Lifetime field of zero.
In the case of a router becoming a host, the system SHOULD also
depart from the all-routers IP multicast group on all interfaces on
which the router supports IP multicast (whether or not they had been
advertising interfaces). In addition, the host MUST ensure that
subsequent Neighbor Advertisement messages sent from the interface
have the Router flag set to zero.

sign-off: Jing Zhang zhangjing@microsoft.com

files/scripts/service_mgmt.sh

@@ -40,7 +40,7 @@ function check_redundant_type()
         ACTIVE_ACTIVE="false"
     fi
     CONFIG_KNOB=`$SONIC_DB_CLI CONFIG_DB hget "MUX_LINKMGR|SERVICE_MGMT" kill_radv`
-    if [[ x"$CONFIG_KNOB" != x"True" ]]; then
+    if [[ x"$CONFIG_KNOB" == x"False" ]]; then
         ACTIVE_ACTIVE='false'
     fi 
     debug "DEVICE_SUBTYPE: ${DEVICE_SUBTYPE}, CONFIG_KNOB: ${CONFIG_KNOB}"