Add a dbtunnel target to elemoine.mk

@sbrunner I had to create a `vars_elemoine.yaml` file for overriding `dbport`. Can you please confirm that it's the way to do it? Thanks.
For me if should be:
```
VARS_FILE = vars_elemoine.yaml
VARS_FILES = vars_geoportailv3.yaml
```
I don't know it it's possible to update the geoportailv3.mk to simplify it ?
For example, in this file:
```
VARS_FILE = vars_elemoine.yaml
```
In geoportailv3.mk:
```
-VARS_FILE ?= vars_geoportailv3.yaml
-VARS_FILES += ${VARS_FILE}
+ifdef VARS_FILE
+VARS_FILES += ${VARS_FILE} vars_geoportailv3.yaml
+else
+VARS_FILE = vars_geoportailv3.yaml
+VARS_FILES += ${VARS_FILE}
+endif
```