Python Ioctx.get_xattrs does not take xattr_name

Fixes bug 5528

Since it returns all xattrs, it does not take an xattr_name. Also,
add unit tests for the Object xattrs methods.

Signed-off-by: Johannes Erdfelt <johannes@erdfelt.com>
Reviewed-by: Josh Durgin <josh.durgin@inktank.com>
test_rados: don't use assert_greater

It's only present in very recent versions of nose, and doesn't provide
much beyond a simple assert.

Signed-off-by: Josh Durgin <josh.durgin@inktank.com>
Reviewed-by: Dan Mick <dan.mick@inktank.com>