Use built-in Pulumi bcrypt

[jemrobinson]

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⤴️ Summary

Replace use of python bcrypt library with the bcrypt functionality built into pulumi.RandomPassword.

Previously we generated admin_password_salt for the DNS server ourselves and stored it as a Pulumi secret. We replace this with use of the bcrypt_hash property of pulumi.RandomPassword which means that we don't need to handle our own salt generation or the combination of salt+password into a bcrypt hash.

🌂 Related issues

Closes #1644

🔬 Tests

Tested on a new SRE deployment

[JimMadge]

LGTM 👍