docs: autodoc_mock_imports for gssapi

Adds mocking of 'gssapi' and 'paramiko[gssapi]' that cannot be installed on ReadTheDocs
builders.
This fixes the ReadTheDocs building problem.
closes #274

run-tests.sh

@@ -83,6 +83,10 @@ Darwin*)
     cd $COMPONENT_NAME &&
     apt update &&
     apt install git -y && # Needed by check-manifest
+    sudo ln -s /usr/bin/krb5-config.mit /usr/bin/krb5-config
+    sudo ln -s /usr/lib/x86_64-linux-gnu/libgssapi_krb5.so.2 /usr/lib/libgssapi_krb5.so
+    sudo apt-get install python-pip libkrb5-dev
+    sudo pip install gssapi
     pip install --force-reinstall ../reana-commons ../reana-db ../pytest-reana &&
     pip install .[all] && # Install test dependencies
     eval $RUN_TESTS"