Modify burnin to support file storage backend
Modify burnin to support file storage backend

This patch does two things:
- adjusts burnin to work with the file storage backend patches
- adds the possibilty to do a burnin for file based instances

Reviewed-by: iustinp


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