Include default schema when checking for migration table

[hundt-corbalt]

instead of checking against all schemas.

Testing: manually ran Migrate with different default schemas and verified that it works and creates two different sets of tables.

PR Checklist

• New automated tests have been written to the extent possible.
• The code has been checked for structural/syntactic validity.
  • AMI/application: a build was performed
  • terraform changes: "terraform plan" checked on every affected environment
• (If applicable) the code has been manually tested on our infrastructure.
  • AMI/application: deployed an a test or dev environment
  • terraform changes: applied to test or dev environment
  • script: run against test or dev environment
• Likely failure points and new functionality have been identified and tested manually.
  Examples:
  • Application manually run in a way that triggers any new branches
  • AMI logged into and changes verified from login shell
• Pull request description includes a description of all the manual steps performed to accomplish the above.

To provide feedback on this template, visit https://docs.google.com/document/d/1YfTv7Amyop5G_8w1c2GJ_Mu-70L0KkZHhm9f9umDi3U/edit

[leslie-corbalt]

LGTM