Revert "fs/basic: multimds"

This reverts commit aea501beb744dd6f6ec2b60678cefc7fae6761b0.

Let's put this in a separate subdir for now.  It won't pass on firefly because
the libcephfs changes aren't there.
multimds: new (separate) suite for multi-mds tests

Eventually we can merge this into the fs suite, but for now it is useful
to segregate the test cases.

Start with a dup of basic and verify sections; add others later.

Signed-off-by: Sage Weil <sage@inktank.com>