Allow manage existing SSM instance

[lnesteroff]

Modified the secure-source-manager-instance module to allow management of an existing instance (eg create repos and manage IAM). Also removed the location attr from the repositories var as it needs to be the same as the instance, so this would save having to enter a location for each repo being created.

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Checklist

I applicable, I acknowledge that I have:

• Read the contributing guide
• Ran terraform fmt on all modified files
• Regenerated the relevant README.md files using tools/tfdoc.py
• Made sure all relevant tests pass

[juliocc]

LGTM except for small comment. See inline