Environment variable check for system tests doc

Related-Bug: #1321682

Change-Id: Ib974d34d15fd94faa024eb760f0961c32572e9ca

docs/conf.py

@@ -15,6 +15,10 @@ import os
 import sys
 
 sys.path.insert(0, os.path.join(os.path.abspath('.'), "..", "nailgun"))
+
+if "SYSTEM_TESTS_PATH" in os.environ:
+    sys.path.append(os.environ.get("SYSTEM_TESTS_PATH"))
+
 autodoc_default_flags = ['members', 'show-inheritance']
 autodoc_member_order = 'bysource'
 

docs/develop.rst

@@ -10,6 +10,7 @@ Development Documentation
    develop/architecture
    develop/sequence
    develop/env
+   develop/system_tests/tree
    develop/nailgun
    develop/module_structure
    develop/fuel_settings

docs/develop/system_tests/tree.rst

@@ -0,0 +1,9 @@
+.. _system-tests:
+
+System Tests
+============
+
+To include documentation on system tests, **SYSTEM_TESTS_PATH**
+environment variable should be set before running *sphinx-build* or
+*make*.
+