Idempotentiate create_tables.sql

[zrhoffman]

What does this PR (Pull Request) do?

• This PR closes Postinstall can no longer be run twice #4984
• This PR makes it so that, even if ./admin load_schema is run more than once (for example, installing TO, with a shared database, on more than 1 host simultaneously), an SQL error should not result.

Which Traffic Control components are affected by this PR?

• Traffic Ops (initial create_tables.sql seed data)

What is the best way to verify this PR?

1. Run the Traffic Ops DB tests

   ./pkg traffic_ops_build;
   cd traffic_ops_db/test/docker;
   cp ../../../dist/traffic_ops-5.0.0-10961.9250b20e.el7.x86_64.rpm traffic_ops.rpm;
   docker-compose up --build;

2. Check that the resultant schema is identical to the one we would have gotten from create_tables.sql previously.

   import_and_dump_schema() {
       output_file="$1";
       <<'DOCKER_COMMANDS' docker run --rm -i \
           -v "$(pwd)/traffic_ops/app/db/create_tables.sql:/create_tables.sql" \
           -e POSTGRES_USER=traffic_ops \
           -e POSTGRES_PASSWORD=twelve \
           -e PGPASSWORD=twelve \
           -e POSTGRES_DB=traffic_ops \
           postgres:9.6-alpine bash >"$output_file";
       trap 'echo "Error on line ${LINENO} of ${0}" >/dev/stderr; exit 1' ERR;
       set -o errexit;
       /docker-entrypoint.sh postgres >/dev/stderr &
       echo 'Waiting for Postgres to start...' >/dev/stderr;
       sleep 10;
       psql -Utraffic_ops -fcreate_tables.sql >/dev/stderr;
       pg_dump -Utraffic_ops;
   DOCKER_COMMANDS
   };
   git checkout origin/master;
   import_and_dump_schema upstream.sql;
   git checkout zrhoffman/postinstall-twice;
   import_and_dump_schema pr.sql;
   wc -l upstream.sql pr.sql; # Should be 4429 lines
   openssl sha256 upstream.sql pr.sql; # Hash should be 794fd7a23fa690eec67f6cd1257a9a8d900ac59edbdc487119ef73023a938d84

The following criteria are ALL met by this PR

• This PR includes tests
• You should never actually run create_tables.sql twice, no documentation necessary
• This PR includes an update to CHANGELOG.md
• This PR includes any and all required license headers
• This PR ensures that database migration sequence is correct
• This PR DOES NOT FIX A SERIOUS SECURITY VULNERABILITY (see the Apache Software Foundation's security guidelines for details)

[rawlinp]

This would be a good test case to add to https://github.com/apache/trafficcontrol/blob/master/traffic_ops_db/test/docker/run-db-test.sh if we can.

[zrhoffman]

DB test added in 9250b20ec8

[zrhoffman]

Rebased to fix a merge conflict in the changelog

[rawlinp]

TODB tests passed