Add ENV vars to set default and all interface roles forward

Add FORWARD_DEFAULT, MGMT_FORWARD, PRIVATE_FORWARD and STORAGE_FORWARD
 to test settings. If FORWARD_DEFAULT is defined it will become the
 default value for {ADMIN,PUBLIC,MGMT,PRIVATE,STORAGE}_FORWARD
 parameters. In it's absence the defaults are.

 ADMIN_FORWARD nat
 PUBLIC_FORWARD nat
 MGMT_FORWARD None (isolated)
 PRIVATE_FORWARD None (isolated)
 STORAGE_FORWARD None (isolated)

Closes-bug: #1275859

Change-Id: I0e5ebbb3269b02e6477323c53f6114086a225000
Merge "Add ENV vars to set default and all interface roles forward"