Fix SyntaxWarning with Python 3.8

Under Python 3.8, the following SyntaxWarning is logged on every
glanceclient invocation:

 SyntaxWarning: "is not" with a literal. Did you mean "!="?
  if kwargs.get('cacert', None) is not '':

Make use of '!=' rather than 'is not' to avoid this warning.

Change-Id: I11ed568932ec5ea0ffee629d6fd434433f262b67

glanceclient/common/http.py

@@ -177,7 +177,7 @@ class HTTPClient(_BaseHTTPClient):
             if kwargs.get('insecure', False) is True:
                 self.session.verify = False
             else:
-                if kwargs.get('cacert', None) is not '':
+                if kwargs.get('cacert', None) != '':
                     self.session.verify = kwargs.get('cacert', True)
 
             self.session.cert = (kwargs.get('cert_file'),