Fixed invalid error message

In setcontext() method of tempest/common/glance_http
module, error message is not formatted property which
leads to error "TypeError: not all arguments converted
during string formatting".

Added appropriate format specifiers to avoid error.

Closes-Bug: 1483230
Change-Id: I48a1f7bf45f563de5959e97ff43b1bc2845feb9f
diff --git a/tempest/common/glance_http.py b/tempest/common/glance_http.py
index 4be3da1..868a3e9 100644
--- a/tempest/common/glance_http.py
+++ b/tempest/common/glance_http.py
@@ -330,7 +330,7 @@
             try:
                 self.context.load_verify_locations(self.ca_certs)
             except Exception as e:
-                msg = 'Unable to load CA from "%s"' % (self.ca_certs, e)
+                msg = 'Unable to load CA from "%s" %s' % (self.ca_certs, e)
                 raise exc.SSLConfigurationError(msg)
         else:
             self.context.set_default_verify_paths()