generic_bird: allow config var GENERIC_BIRD_INTERNAL_PORT in template…

… expansion

To fix this error in the proxy:

requests.exceptions.InvalidURL: Failed to parse:
http://generic_bird:${GENERIC_BIRD_INTERNAL_PORT}/wps
?service=WPS&version=1.0.0&request=GetCapabilities

birdhouse/optional-components/generic_bird/default.env

@@ -15,6 +15,7 @@ export GENERIC_BIRD_POSTGRES_IMAGE="postgres:10.12"
 OPTIONAL_VARS="
   $OPTIONAL_VARS
   \$GENERIC_BIRD_NAME
+  \$GENERIC_BIRD_INTERNAL_PORT
 "
 
 # add any component that this component requires to run