Allow network-less instances
Allow network-less instances

Currently there's no way to specify no NICs for an instance, even though
this is a supported configuration.

The patch adds a --no-nics option to gnt-instance add.

Reviewed-by: amishchenko

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