openbmctool: Add import simplejson

Without the simplejson libary, the CertificateString can't be
serialized from byte arrary successfully. It's better to add
the explict "import simplejson" to avoid this kind of error.

The error message without the simplejson library
  TypeError: b'-----BEGIN CERTIFICATE... is not JSON serializable

Signed-off-by: Alvin Wang <alvinwang@msn.com>
Change-Id: I50fb0c4a3f64bc8481d8162141292ea02a2eb2bc
diff --git a/thalerj/openbmctool.py b/thalerj/openbmctool.py
index 2d3fb3d..f93c7c2 100755
--- a/thalerj/openbmctool.py
+++ b/thalerj/openbmctool.py
@@ -16,6 +16,7 @@
 """
 import argparse
 import requests
+import simplejson
 import getpass
 import json
 import os