Allow setting create_temp_database to false to stop temp db creation

TestSessionEnvironment by default will create a temporary database for
use with behat tests, but this may not be required.

This provides the ability to set configuration like the following to
override that default and force it to not create a temp session db:

e.g. in your behat.yml:

default:
...

  extensions:
    ...
    SilverStripe\BehatExtension\Extension:
      create_temp_database: false