add dockerfile_fallback for etcd

[ashwindasr]

Follows #5002

Add fallback for etcd

[openshift-ci]

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[openshift-bot]

Validating 1 file(s)...
Validating images/cluster-etcd-operator.yml...
Traceback (most recent call last):
File "/usr/local/bin/validate-ocp-build-data", line 33, in
sys.exit(load_entry_point('rh-ocp-build-data-validator==0.2.5', 'console_scripts', 'validate-ocp-build-data')())
File "/usr/local/lib/python3.9/site-packages/validator/main.py", line 87, in main
validate(f, args.exclude_vpn, args.schema_only)
File "/usr/local/lib/python3.9/site-packages/validator/main.py", line 23, in validate
support.fail_validation(msg, parsed)
File "/usr/local/lib/python3.9/site-packages/validator/support.py", line 14, in fail_validation
raise exceptions.ValidationFailed(msg)
validator.exceptions.ValidationFailed: Schema mismatch: images/cluster-etcd-operator.yml
Returned error: Additional properties are not allowed ('dockerfile_fallback' was unexpected)