[18.0-fr2] Propagate top-level extraMounts to Horizon and Keystone

[openshift-cherrypick-robot]

This is an automated cherry-pick of #1441

/assign olliewalsh

[openshift-ci]

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[openshift-ci]

@openshift-cherrypick-robot: The following tests failed, say /retest to rerun all failed tests or /retest-required to rerun all mandatory failed tests:

Test name	Commit	Details	Required	Rerun command
ci/prow/functional	44c6ee1	link	true	/test functional
ci/prow/precommit-check	44c6ee1	link	true	/test precommit-check
ci/prow/openstack-operator-build-deploy-kuttl-4-18	44c6ee1	link	true	/test openstack-operator-build-deploy-kuttl-4-18
ci/prow/openstack-operator-build-deploy-kuttl	44c6ee1	link	true	/test openstack-operator-build-deploy-kuttl

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[olliewalsh]

Will need a manual cherry-pick, keystone PR was not backported