Fix params.defaultConfig overwriting customisations #499

Job	Run time
linting (lint_jsonnet)	5s
linting (lint_yaml)	6s
test (capacity-alerts)	52s
golden (capacity-alerts)	52s
test (release-4.11)	50s
linting (lint_adoc)	9s
editorconfig	9s
golden (release-4.11)	48s
test (remote-write)	48s
golden (remote-write)	48s
test (user-workload-monitoring)	45s
golden (user-workload-monitoring)	46s
golden (capacity-alerts-with-node-labels)	47s
test (capacity-alerts-with-node-labels)	48s
golden (vsphere)	52s
test (vsphere)	46s
golden (custom-rules)	58s
test (custom-rules)	46s
golden (release-4.12)	1m 0s
test (release-4.12)	46s
golden (release-4.13)	47s
test (release-4.13)	48s
golden (team-routing)	53s
test (team-routing)	1m 0s
	17m 9s

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