Proposal: for Rails project automatically setup variables DB_MIGRATE and DB_INITIALIZE

When using auto DevOps for Rails project, it still requires to setup DB_MIGRATE and DB_INITIALIZE variables. It would be nice to offer users an "out-of-the-box" solution in this case.

For all Rails projects, the value of those variables will be the same, if users are using non-Rails-way config, they should have an ability to override this variable.

/cc @danielgruesso