Fixed downsampling for mutect to use Broad default when max_coverage_dep...

...th set to zero

Hi Brad,
Noticed that my previous code would set downsampling for MuTect to 200 in case max_coverage_depth is set to zero. This fix reverts to Broad default (1.5k at the moment). Comments and feedback welcome.
Should this be `max(1500` to match the default?