Turn on debug mode by default

[mtreinish]

By default cliff suppresses stack traces on unhandled exceptions in order
to keep the output cleaner for users. But in practice this has caused more
confusion for stestr users since often the error messages aren't really
descriptive enough to indicate what's going wrong (or that it's even a
bug in stestr). This is often because exception messages are written
assuming the context of the stack trace for debugging, not to be user
facing messages. Cliff offers the --debug flag to enable printing the
full stack traces. This commit turns that flag on by default when stestr
initializes the cliff App object to ensure that it always prints the
full stack trace on an unhandled exception.

Fixes #220

[coveralls]

Coverage increased (+0.02%) to 65.314% when pulling 300f0b2 on debug-by-default into 18d6150 on master.

[masayukig]

LGTM. This can provide much more useful information to users. But I also think we should care #219 case. Maybe, another patch is needed?

When I saw this PR at the first time, I felt this is not good behavior, we should suppress stack traces to users as possible. But after I read #219 and #220 , it's OK as a kind of workaround.

So, I think an ideal behavior is that we catch all errors (in cliff or stestr), and then, print more useful message like @aspiers said in #220 (comment) .