User-agent header shouldn't start with a space

If the application name, sys.argv[0], is an empty string, like when
called from the Ansible os_network module, then keystoneauth (with
the new requests 2.11.0) will raise an exception saying:

Invalid return character or leading space in header: User-Agent

This comes from keystoneauth trying to prepend the empty module
name to the default user agent, which leads to it sending a
User-Agent header that starts with a blank space. This was fine
until the new requests 2.11.0 was released which errors if header
values start with a leading space.

With this change, if sys.argv[0] is an empty string we'll just use
the DEFAULT_USER_AGENT and not try to prepend a calling module.

Closes-Bug: #1611426

Change-Id: I56d3e352dce7628add0479b3333a880700844ebc
Signed-off-by: Matt Mulsow <mamulsow@us.ibm.com>

keystoneauth1/session.py

@@ -155,6 +155,9 @@ def _determine_user_agent():
         # sys.argv is empty, usually the Python interpreter prevents this.
         return None
 
+    if not name:
+        return None
+
     name = os.path.basename(name)
     if name in ignored:
         name = _determine_calling_package()

keystoneauth1/tests/unit/test_session.py

@@ -146,9 +146,11 @@ class SessionTests(utils.TestCase):
         # If sys.argv[0] is an empty string, then doesn't fail.
         with mock.patch.object(sys, 'argv', ['']):
             session = client_session.Session()
-            # NOTE(blk-u): This isn't working right, see bug 1611426
-            self.assertRaises(exceptions.UnknownConnectionError, session.get,
-                              self.TEST_URL)
+            resp = session.get(self.TEST_URL)
+            self.assertTrue(resp.ok)
+            self.assertRequestHeaderEqual(
+                'User-Agent',
+                client_session.DEFAULT_USER_AGENT)
 
     def test_http_session_opts(self):
         session = client_session.Session(cert='cert.pem', timeout=5,