Improve mysqld image defaulting

[vyzigold]

This adds mysqld exporter image to the telemetry webhook (before this it was part only of the ceilometer webhook)

[softwarefactory-project-zuul]

Build failed (check pipeline). Post recheck (without leading slash)
to rerun all jobs. Make sure the failure cause has been resolved before
you rerun jobs.

https://softwarefactory-project.io/zuul/t/rdoproject.org/buildset/039b35fea7af4d1982e7f2139664b5e2

✔️ openstack-k8s-operators-content-provider SUCCESS in 1h 56m 34s
✔️ podified-multinode-edpm-deployment-crc SUCCESS in 1h 18m 03s
✔️ telemetry-operator-multinode-autoscaling-tempest SUCCESS in 1h 16m 00s
❌ telemetry-operator-multinode-default-telemetry FAILURE in 1h 05m 39s
✔️ functional-tests-on-osp18 SUCCESS in 1h 38m 29s (non-voting)
❌ functional-logging-tests-osp18 FAILURE in 30m 55s (non-voting)
✔️ functional-graphing-tests-osp18 SUCCESS in 1h 05m 43s (non-voting)
✔️ functional-metric-verification-tests-osp18 SUCCESS in 1h 09m 32s (non-voting)

[vyzigold]

recheck - operator restarted.

[jlarriba]

/lgtm

[openshift-ci]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: jlarriba, vyzigold

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[vyzigold]

/retest

[openshift-ci]

@vyzigold: The following test 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/telemetry-operator-build-deploy-kuttl	05071c5	link	true	/test telemetry-operator-build-deploy-kuttl

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