Environment variables, third iteration

See issue #120, #130, #134

Added environment variables:

SNABB_TEST_VHOST_USER_SOCKET filename of the vhost_user socket file.
SNABB_TEST_INTEL10G_PCI_ID PCI ID of an Intel 82599 network device.
SNABB_TEST_INTEL10G_PCIDEVA PCI ID of the first Intel 82599 network device.
SNABB_TEST_INTEL10G_PCIDEVB PCI ID of the second Intel 82599 network device.