Offer to create _ss_environment on sspak install.

[sminnee]

As part of creating a new environment during 'sspak install', we should deal with the situation where an _ss_environment.php file doesn't yet exist.

If the DB details are incomplete, and _ss_environment.php doesn't exist, offer to create it. This can be an interactive command-line too, and it may be part of sspak, or it may be part of "sake dev/build".