Make dbCollections parameter compatible to both string and json struct #330

Job	Run time
test-backup-restore-with-custom-config (helm, standalone, milvus-backup)	6m 48s
test-backup-restore-with-custom-config (helm, standalone, milvus-bucket)	6m 26s
test-backup-restore-with-custom-config (helm, standalone, file, milvus-backup)	6m 58s
test-backup-restore-with-custom-config (helm, standalone, file, milvus-bucket)	7m 55s
test-backup-restore-cli (docker-compose, standalone, standalone)	5m 20s
test-backup-restore-cli (docker-compose, standalone, cluster)	5m 23s
test-backup-restore-cli (docker-compose, cluster, standalone)	5m 27s
test-backup-restore-cli (docker-compose, cluster, cluster)	5m 28s
test-backup-restore-cross-version (docker-compose, standalone, standalone)	5m 26s
test-backup-restore-api (docker-compose, standalone)	13m 24s
test-backup-restore-api (docker-compose, cluster)	15m 36s
test-backup-restore-api (helm, standalone)	16m 24s
	1h 40m 35s

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