make test: python interpreter customization

Allow using custom python interpreter via 'PYTHON' env variable.
E.g. env PYTHON=pypy make test.
Get latest pip automatically to properly function in environments,
where old pip is available and a proxy is required. This allows
testing pypy in a centos vagrant-provided vm.

Change-Id: I67a658fc927303468cc67f0ac192317ca2907625
Signed-off-by: Klement Sekera <ksekera@cisco.com>
Signed-off-by: Damjan Marion <damarion@cisco.com>
diff --git a/Makefile b/Makefile
index 8964329..133c3d4 100644
--- a/Makefile
+++ b/Makefile
@@ -252,6 +252,7 @@
 	  VPP_TEST_INSTALL_PATH=$(BR)/install-$(2)-native/ \
 	  LD_LIBRARY_PATH=$(BR)/install-$(2)-native/vpp/lib64/ \
 	  EXTENDED_TESTS=$(EXTENDED_TESTS) \
+	  PYTHON=$(PYTHON) \
 	  $(3)
 endef