meta-phosphor: make discovery bbclass use override syntax

In the spirit of consistency with the new upstream override
syntax, change phosphor-discovery-service.bbclass to use it
and make tree-wide changes as appropriate.

After the change checked on Witherspoon on QEMU:
```
root@witherspoon:/etc/avahi/services# ls
obmc_console.service  obmc_redfish.service  obmc_rest.service
```

Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
Change-Id: Ib1fe3cce57a0130378af789abd83b457e0c3a318
diff --git a/meta-ibm/recipes-phosphor/interfaces/bmcweb_%.bbappend b/meta-ibm/recipes-phosphor/interfaces/bmcweb_%.bbappend
index 659eafd..8e06233 100644
--- a/meta-ibm/recipes-phosphor/interfaces/bmcweb_%.bbappend
+++ b/meta-ibm/recipes-phosphor/interfaces/bmcweb_%.bbappend
@@ -5,5 +5,5 @@
 
 inherit obmc-phosphor-discovery-service
 
-REGISTERED_SERVICES_${PN} += "obmc_redfish:tcp:443:"
-REGISTERED_SERVICES_${PN} += "obmc_rest:tcp:443:"
+REGISTERED_SERVICES:${PN} += "obmc_redfish:tcp:443:"
+REGISTERED_SERVICES:${PN} += "obmc_rest:tcp:443:"