Pin docplex version in constraints.txt for tutorials CI job

The recent new release of docplex has broken the released version of
aqua. There is a fix in aqua in-progress qiskit-community/qiskit-aqua#1420, but
until that is included in a release we won't be able to use the latest
version of docplex in the tutorials CI job. This commit adds a pin on
the version of docplex used in the constraints file to always use a
known working version to unblock CI.

constraints.txt

@@ -0,0 +1 @@
+docplex==2.15.194