SYS-665 nexus helm chart - add postgres

[instantlinux]

Summary of Changes

• Add postgres subchart to nexus helm chart
• Define database variables and secrets for nexus database

Also:

• fix ansible role kubernetes, which had a data-type error for cplane variable.

Why is this change being made?

Orientdb database phased out by vendor; postgres supplies the best fault-tolerant option now available (notwithstanding online statements to the contrary, Community Edition of Sonatype Nexus does support PostgreSQL: this helm chart came up and populated the blank database literally on first try).

How was this tested? How can the reviewer verify your testing?

Local testing.

Completion checklist

• [x] The pull request is linked to all related issues
• [x] This change has unit test coverage
• [x] Documentation has been updated
• [x] Dependencies have been updated and verified