Add jsonschema to requirements.txt

This commit adds jsonschema to the requirements list. It is used in
the glance api v2 testing, however was previously not included on the
requirements list. This commit corrects the oversight.

Change-Id: I6615d07734f693de665b7d89fc20a38a4a1a9e63
diff --git a/requirements.txt b/requirements.txt
index 06aa9f3..ee754dc 100644
--- a/requirements.txt
+++ b/requirements.txt
@@ -3,6 +3,7 @@
 anyjson
 nose
 httplib2>=0.7.0
+jsonschema>=1.0.0,!=1.4.0,<2
 testtools>=0.9.32
 lxml
 boto>=2.2.1