Vf/4018/env variables

See issue #120.

Several hardcoded paths and values now can be defined as environment variables:

* SNABB_VFIO_DRIVER
  Default value "/sys/bus/pci/drivers/vfio-pci"

* SNABB_VFIO_IOMMU_GROUPS
  Default value "/sys/kernel/iommu_groups"

* SNABB_PCI_DEVICE
  Default value "/sys/bus/pci/devices"

* SNABB_TEST_PCI_ID
  Default value "0000:01:00.0"

* SNABB_HUGEPAGES
  Default value "/proc/sys/vm/nr_hugepages"

* SNABB_MEMINFO
  Default value "/proc/meminfo"

'src/doc/hacking.md' file added with variables listing and examples.