Update to the latest version of Vault

[robmorgan]

I hit quite a few yak shaves, trying to update this repo to the latest version of Vault, so I've tried to include the minimum here to make the tests pass:

• Upgrade Vault to 1.5.4.
• Output the Managed Instance Group Name and change the tests to use this value. Google changed their API and the tests were failing.
• Hardcode the latest working Vault Enterprise Download URL. HashiCorp publishes these publically now, so there's no need to cloak the URL using env vars.
• Bump the Bastion Ubuntu image to an LTS version.
• Update Packer templates to support Packer 1.6.x.
• Updated the CircleCI config to prefer Terraform 0.11.x and install Packer.
• Fix a few tiny provider issues in preparation for the Terraform 0.13 upgrade.
• Disabled the Consul DNS checks. Filed DNS functionality failing #56 for that.

[hashicorp-cla]

All committers have signed the CLA.