Add support for extraContainers and extraInitContainers (#877)

charts/mariadb/Chart.yaml

charts/mariadb/README.md

charts/mariadb/RELEASENOTES.md

charts/mariadb/templates/statefulset.yaml

charts/mariadb/values.yaml

charts/mongodb/Chart.yaml

charts/mongodb/README.md

charts/mongodb/RELEASENOTES.md

charts/mongodb/templates/statefulset.yaml

charts/mongodb/values.yaml

charts/postgres/Chart.yaml

charts/postgres/README.md

charts/postgres/RELEASENOTES.md

charts/postgres/templates/statefulset.yaml

charts/postgres/values.yaml