remove basedir/testdir distinction

We should never run with a conflicting testdir in the basedir, and the
code to do this is confusing and buggy.  Go back to a single testdir and
simple checks.

Signed-off-by: Sage Weil <sage@inktank.com>
(cherry picked from commit 5acc57f5ad6ed14b3e1c89166d2571cf302a467c)

Conflicts:

	README.rst
	teuthology/misc.py