WP Install fails due to soft link creation issues

[kcristiano]

At the St Louis sprint, some uses experienced issues with civibuild create wp-master Traced this to creating a soft link here https://github.com/civicrm/civicrm-buildkit/blob/master/app/config/wp-demo/download.sh#L15

This appears to be intermittent, and looks to be permissions related to running Virtual Box on Windows.

As wp-cli does not need yml file in each site, for the Vagrant buildkit we can add a single yml file at ~/.wp-cli/config.yml see theb docs:
https://make.wordpress.org/cli/handbook/config/#config-files

We will need a corresponding patch to buildkit since the fact that we can create a single yml here will not address the code that shows the error. Perhaps we should test to see if ~/.wp-cli/config.yml exists before creating the soft link.