Replace all constant definitions with re-exports
Replace all constant definitions with re-exports

Remove all constant definitions in 'lib/constants.py' and re-export
the module 'lib/_constants.py', which contains the generated
constants.

Signed-off-by: Jose A. Lopes <0fb9c4f8511df4ec2ae28af879e9d462f6218a2c@google.com>
Reviewed-by: Michele Tartara <05f7077a2c6cbbdc4af4ba42e3ef89a0df846179@google.com>

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