Make osprofiler dependency really "soft"

In a37b5fe562 osprofiler was
changed to a soft requirement. However, if it is missing then
mistralclient will currently fail with:

AttributeError: 'NoneType' object has no attribute 'get_trace_id_headers'

This change checks to see if it was successfully imported before
it is used.

Depends-On: I1b61bd1de6811a5f65c2375dde956aafe33445b2
Change-Id: Ia742a4e55b35c5a787e4606ff2f91425dd156e5e

mistralclient/api/httpclient.py

@@ -128,7 +128,8 @@ class HTTPClient(object):
         if target_auth_uri:
             headers['X-Target-Auth-Uri'] = target_auth_uri
 
-        # Add headers for osprofiler.
-        headers.update(osprofiler_web.get_trace_id_headers())
+        if osprofiler_web:
+            # Add headers for osprofiler.
+            headers.update(osprofiler_web.get_trace_id_headers())
 
         return headers