Change environment test to check for 'physical'

Instead of checking against different types of virtual
hardware, we can test against 'physical', which should
mean we're not in Vagrant. If this doesn't work, we can
also set a custom fact in the Vagrantfile.
Add quickstart_env_is_staging()

Add an environment variable when Quickstart is provisioned
depending on the current environment. We can use that
in PHP to for our `quickstart_env_is_staging()` function.

Fixes #346
Merge pull request #347 from Automattic/issue-346

Add `quickstart_env_is_staging()`