[sflow] Fix race-condition seen with mVRF configured (sonic-net#6102)

Under certain conditions, the sFlow service can start before
interface configurations are sucessfully applied. This will
cause hsflowd to get a socket error.

This fix ensures all interface configurations are successfully
applied before the sFlow service (hsflowd) starts.

During testing we saw this error from hsflowd if interface configs were not successfully applied before hsflowd started.

    ERR sflow#hsflowd: socket sendto error: Network is unreachable

no FLOW samples can be seen. This can be consistently reproducible if you force sFlow service to start before interface-config.service.

Signed-off-by: Garrick He <garrick_he@dell.com>

files/build_templates/sflow.service.j2

@@ -1,7 +1,7 @@
 [Unit]
 Description=sFlow container
 Requisite=swss.service
-After=swss.service syncd.service
+After=swss.service syncd.service hostcfgd.service
 Before=ntp-config.service
 StartLimitIntervalSec=1200
 StartLimitBurst=3