add vt in docker + optional process

[tdelhomme]

vt is a tool set for short variant discovery in genetic sequence data. Here is the official GitHub repository. A good practice when run platypus is to add a vt step to normalize variants (needed for Annovar for instance).

vt program should be include in the Dockerfile in order to add an optional step for vt after variant calling by platypus.

[tdelhomme]

I close the issue, a best idea is to keep separated the vt normalization (see corresponding nextflow pipeline). Instead, I will open an issue to add a compression/indexation process.