Convert cmdlib.py to _FieldSet
Convert cmdlib.py to _FieldSet

This patch converts the current usage of _CheckOutputFields to the
FieldSet class, but it doesn't start to use its variable matching
features.

Reviewed-by: ultrotter

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