katib: Add KATIB_POSTGRESQL_SSL_MODE variable of Katib DB Manager

[ckcd]

This follow-up PR reflects a change from kubeflow/katib#2266, which introduced a new environment variable option to allow users to specify the value of Postgres sslmode.

It updates Katib DB Manager part of Environment Variables for Katib Components.

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[google-oss-prow]

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