discover shell files

when attempting to integrate bashate into grenade, devstack-gate,
and devstack via tox, one thing that really stood out was that we
have to provide an explicit file list, which ends up more different
than you think.

This changes the bashate behavior to do a discovery when no files
are passed on the command line. The current set of files that
devstack and grenade need are encoded, but I think we should look
at moving those all to .sh extension in the future, then we can
remove these extra parts.

Change-Id: I9423a5ad3496d77e8d13afef4e539ffb3e81015a