Ensemble fix

Hi Brad,
Here are a few fixes to alleviate problems with empty variant files, re. https://github.com/chapmanb/bcbio-nextgen/issues/463

I've implemented a few things:
- Tag sample names into final vcf files
- In ensemble calling if no file contains variants, produce just an empty .vcf and bail out
- Fixed improper header in ``write_empty_vcf`` that caused problems with PyVCF
- In FreeBayes hard filtering, if no variants exist, skip
- Implemented sample names in ``write_empty_vcf``