Steps to reproduce:

1. Run wp-now start in one project.
2. Run wp-now start in a second project.
3. Open the localhost URL for the first project and observe it redirect to the second project.

The problem is that ~/.wp-now/wordpress-versions/latest has a hardcoded file:

$ cat playground-consts.json
{"WP_HOME":"http://127.0.0.1:8883","WP_SITEURL":"http://127.0.0.1:8883","WP_AUTO_UPDATE_CORE":true}

We might want to virtualize wp-config.php entirely so we can define constants on the fly, etc. Alternatively, the configuration details should live in the project's wp-content directory (e.g. ~/.wp-now/wp-content/gutenberg-f423215415c76d74a97bec1bcfb0e3d4afe8d8e5)

cc @adamziel

Done is:

• It's possible to run multiple wp-now servers at the same time, and not have them conflict with their configuration.
• We've added tests to verify this won't happen again in the future (related Local Environment: Add tests for first and second run step behaviors for each mode #312)