Allow override of packages to be installed

[gibbsoft]

• add packages parameter to allow the override of packages to be
  installed. For example, some packages in the list may need to be
  omitted if they are installed via another module in the catalog.

• the default package list is set to ensure existing behaviour is
  preserved.

[jetstack-bot]

Hi @gibbsoft. Thanks for your PR.

I'm waiting for a jetstack member to verify that this patch is reasonable to test. If it is, they should reply with /ok-to-test on its own line. Until that is done, I will not automatically test new commits in this PR, but the usual testing commands by org members will still work. Regular contributors should join the org to skip this step.

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