Description of problem:

Various places, including CI/QE and the engine appliance, maintain answer files
for engine-setup, in order to achieve unattended setup. This breaks whenever
a new question is added to engine-setup.

It will be helpful if we add an option called '--accept-defaults' which will
make engine-setup prompt only for questions for which it does not supply a
default, accepting the defaults for all of the others.

We should consider doing this change also in related tools - engine-cleanup,
ovirt-engine-rename, hosted-engine setup, etc. We will probably need to
change otopi, perhaps even only otopi.

This is of course also useful for normal interactive use, in case the user
knows that the defaults are ok.