Add function to obtain "forthcoming" field version
Add function to obtain "forthcoming" field version

This function renders fields into their "forthcoming" version.
Usually, this just means making the field optional.

Signed-off-by: Klaus Aehlig <4ce51de36d9068cd7db2eec1151e0f3f95419008@google.com>
Reviewed-by: Petr Pudlak <9d19625f8ee825a2c398462ed335cbcf98f79c32@google.com>

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