Pin docplex version in constraints.txt for tutorials CI job (#5366)

* 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.

* Remove windows pip cache

The windows CI images seem to have been updated and now the pip cache
step can't find the local pip cache to upload to azure. To unblock CI
this commit just removes the windows caching from CI. It will add some
execute time, but that's still better than not working.

azure-pipelines.yml

@@ -359,22 +359,12 @@ stages:
             python.version: '3.8'
       variables:
         QISKIT_SUPPRESS_PACKAGING_WARNINGS: Y
-        PIP_CACHE_DIR: $(Pipeline.Workspace)/.pip
         QISKIT_TEST_CAPTURE_STREAMS: 1
       steps:
         - task: UsePythonVersion@0
           inputs:
             versionSpec: '$(python.version)'
           displayName: 'Use Python $(python.version)'
-        - task: Cache@2
-          inputs:
-            key: 'pip | "$(Agent.OS)" | "$(python.version)" | "$(Build.BuildNumber)"'
-            restoreKeys: |
-              pip | "$(Agent.OS)" | "$(python.version)"
-              pip | "$(Agent.OS)"
-              pip
-            path: $(PIP_CACHE_DIR)
-          displayName: Cache pip
         - bash: |
             set -e
             python -m pip install --upgrade pip setuptools wheel virtualenv

constraints.txt

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