Fix analytics UT failure due to broken cassandra-driver

cassandra-driver depends on Cython package. Currently, installing
cassandra-driver<=3.7.0 from PyPi or Github master will install the
latest version of Cython rather than a compatible version. The latest
version of Cython (0.25) has broken the cassandra-driver.The version of
Cython that works with cassandra-driver is 0.24.1

Fixed the issue by installing Cython==0.24.1 before installing the
cassandra-driver.

Change-Id: I6934dc1cbed31e82ebace180e781637328e7be34
Closes-Bug: #1637070

src/opserver/test-requirements.txt

@@ -9,6 +9,7 @@ flexmock==0.9.7
 kafka-python==0.9.2
 kazoo
 stevedore
+Cython==0.24.1
 cassandra-driver==3.0.0
 amqp==1.4.1
 kombu==3.0.9