Search `Vagrantfile.local.yml` relative to the `Vagrantfile`

[ocean90]

… and load Vagrantfile.local.yml only if the file exists.

Fixes an error when running vagrant global-status --prune

$ vagrant global-status --prune
There was an error loading a Vagrantfile. The file being loaded
and the error message are shown below. This is usually caused by
a syntax error.

Path: /Users/Dominik/Documents/webdev/appflow/Vagrantfile
Line number: 0
Message: Errno::ENOENT: No such file or directory @ rb_sysopen - Vagrantfile.local.yml