reworked php7.3 stack

* replace previous stack php7 variables
* properly set stack php = php7.2 and php73 = php7.3
* rename upstream php72 and php73 to avoid issues when migrating from EEv3
* use unix socket in nginx upstream instead of TCP/IP

wo/cli/templates/wpcommon.mustache

@@ -4,7 +4,7 @@
 location = /wp-login.php {
     limit_req zone=one burst=1 nodelay;
     include fastcgi_params;
-    fastcgi_pass php;
+    fastcgi_pass php72;
 }
 # Disable wp-config.txt
 location = /wp-config.txt {