Some small fixes to utils.GenerateTable
Some small fixes to utils.GenerateTable

This adds:
  - fix the case when a dumb caller didn't stringify its values
  - explicitly raise a ProgrammerError in case a field is missing from
    the headers dictionary

Reviewed-by: imsnah


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