Re-enable stacktracing when --debug is used

Commit 1f89beb609 introduced the behaviour
that a stacktrace is printed if an exception is encountered.

This helped make the client more supportable:

 $ glance --debug image-list
  .
  .
  .
  File "glanceclient/common/http.py", line 337, in get_http_client
    xxx
 NameError: global name 'xxx' is not defined
 global name 'xxx' is not defined

The behaviour was lost at some point. This patch re-enables it.

Change-Id: I25fc8624797909d606590747f54b9cf649ade079
Closes-bug: 1563830

glanceclient/common/utils.py

@@ -467,6 +467,14 @@ def endpoint_version_from_url(endpoint, default_version=None):
         return None, default_version
 
 
+def debug_enabled(argv):
+    if bool(env('GLANCECLIENT_DEBUG')) is True:
+        return True
+    if '--debug' in argv or '-d' in argv:
+        return True
+    return False
+
+
 class IterableWithLength(object):
     def __init__(self, iterable, length):
         self.iterable = iterable