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.

e.g. in your behat.yml:

```
default:
...

  extensions:
    ...
    SilverStripe\BehatExtension\Extension:
      create_temp_database: false
```
Merge pull request #75 from halkyon/disable_create_database

Allow setting create_temp_database to false to stop temp db creation