LXC: add virtualenv to LXC_BASE_PACKAGES

Commit 09a40625 adds virtualenv dependency.  BTW remove deprecated
--no-site-packages.  Not having access to global site-packages is now the
default behavior.

Signed-off-by: Markus Heiser <markus.heiser@darmarit.de>

utils/makefile.python

@@ -30,7 +30,6 @@ PYLINT_RC ?= .pylintrc
 TEST_FOLDER  ?= ./tests
 TEST         ?= .
 
-VTENV_OPTS   = "--no-site-packages"
 PY_ENV       = ./$(LXC_ENV_FOLDER)local/py$(PY)
 PY_ENV_BIN   = $(PY_ENV)/bin
 PY_ENV_ACT   = . $(PY_ENV_BIN)/activate
@@ -41,6 +40,7 @@ ifeq ($(OS),Windows_NT)
   PY_ENV_ACT = $(PY_ENV_BIN)/activate
 endif
 
+VTENV_OPTS ?=
 ifeq ($(PYTHON),python)
   VIRTUALENV   = virtualenv
 else