Convert swagger release fixture to env var (#14940)

`pytest awx/main/tests/docs --release=$(VERSION_TARGET)`
where --release is required breaks test discovery and running in vscode (from within the container)

awx/main/tests/docs/conftest.py

@@ -1,13 +1,8 @@
 from awx.main.tests.functional.conftest import *  # noqa
+import os
+import pytest
 
 
-def pytest_addoption(parser):
-    parser.addoption("--release", action="store", help="a release version number, e.g., 3.3.0")
-
-
-def pytest_generate_tests(metafunc):
-    # This is called for every test. Only get/set command line arguments
-    # if the argument is specified in the list of test "fixturenames".
-    option_value = metafunc.config.option.release
-    if 'release' in metafunc.fixturenames and option_value is not None:
-        metafunc.parametrize("release", [option_value])
+@pytest.fixture()
+def release():
+    return os.environ.get('VERSION_TARGET', '')