validate machine var, remove migrate_db prompt

Added a validation on the machine variable, expected to be passed in
from the command line now, so that the worker play only works on
machines named "util" and the app play on machines named "app".
Otherwise it would be too easy to mix those up.

Now that we're using -e command line options, it's easier to use this to
specify the "modify_db" variable than prompt_vars.  Having every install
block on this question before was such a hassle!  Now we just won't
migrate unless you explicitly say so on the ansible command with
"migrate_db=yes" or "migrate_only=yes".