sos/sosreport.py: more py3/py2 compatability changes

Signed-off-by: Adam Stokes <adam.stokes@ubuntu.com>
sos plugins filesyste, xfs, python2/python3 compatbility

Signed-off-by: Adam Stokes <adam.stokes@ubuntu.com>
Increase plugin name padding in list_plugins output

openstack-* is longer than the allotted 15 spaces, increased to 20 to make
things line up again.

Signed-off-by: Adam Stokes <adam.stokes@ubuntu.com>