TT#81700 fix warning errors in jenkins

Since we are mounting in read-only mode the code, we are getting
warnings from pytest executions not able to generate .pytest_cache
directory.

Just define CACHE_DIR in a volumen that is mounted as read-write

Change-Id: I115f4ebcff78172fa809d0a9fdba02f923fb6ecb

Makefile

@@ -1,5 +1,7 @@
 VAR_DIR ?= /var/lib/repoapi
 RESULTS ?= ./reports
+CACHE_DIR ?= $(RESULTS)/.pytest_cache
+
 # do nothing by default
 all:
 
@@ -19,10 +21,12 @@ venv_dev: requirements/dev.txt
 
 test: test_templates
 	RESULTS=$(RESULTS) pytest-3 -ra --junitxml=$(RESULTS)/junit.xml \
+		-o cache_dir=$(CACHE_DIR) \
 		--cov=. --cov-report=xml:$(RESULTS)/coverage.xml --pep8
 
 test_pylint:
 	RESULTS=$(RESULTS) pytest-3 --junitxml=$(RESULTS)/junit.xml \
+		-o cache_dir=$(CACHE_DIR) \
 		--pylint --pylint-rcfile=pylint.cfg --pylint-jobs=4
 
 test_templates: