KVM nic script: enable interface forwarding
KVM nic script: enable interface forwarding

If forwarding is enabled globally this is a no-op. If instead it's
enabled only for some special interfaces where instance traffic has to
go to/comes from (for example a gre tunnel) then it's useful to
explicitely enable it for the instances interfaces as well.

Signed-off-by: Guido Trotter <f62ca4d5358bdaeef0e50e57f9f7e231de63727e@google.com>
Reviewed-by: Michael Hanselmann <68b67885ff74281219b4a4be33cc20fd45078d7c@google.com>

leshchevds/ganeti,mbakke/ganeti,apyrgio/ganeti,andir/ganeti,leshchevds/ganeti,yiannist/ganeti,dimara/ganeti,apyrgio/snf-ganeti,bitemyapp/ganeti,grnet/snf-ganeti,onponomarev/ganeti,apyrgio/ganeti,ganeti-github-testing/ganeti-test-1,andir/ganeti,ganeti/ganeti,yiannist/ganeti,grnet/snf-ganeti,ganeti/ganeti,yiannist/ganeti,ganeti/ganeti,onponomarev/ganeti,ganeti-github-testing/ganeti-test-1,mbakke/ganeti,andir/ganeti,mbakke/ganeti,bitemyapp/ganeti,apyrgio/snf-ganeti,dimara/ganeti,leshchevds/ganeti