Prior to this commit, the devtools used bean factory post processors to
configure the environment with custom, development-time properties. This
meant that the environment was configured as part of the application
context being refreshed. Crucially, this happened after any property
conditions were evaluated making it impossible for the devtools to
change the default auto-configuration behaviour for a bean or
configuration class that was conditional on a property.

This commit moves the configuration of the environment into an
ApplicationListener that listens for the
ApplicationEnvironmentPreparedEvent which is published as soon as the
Environment has been prepared and before the application context is
refreshed.

Closes gh-3726