[multipath] fix add_cmd_output usage

Signed-off-by: Bryn M. Reeves <bmr@redhat.com>
[utilities,plugin] check for return status 126

A return status of 126 indicates 'found but not executable'. This
is returned from timeout for some malformed commands when running
as a normal user but not when running as root, e.g.:

  Running 1/1: multipath...
[plugin:multipath] collecting path '/etc/multipath/'
[plugin:multipath] unpacked command tuple: ('['multipath -l', 'multipath -v4 -ll']', 'None', 'None', 300, 'None')
[plugin:multipath] collecting output of '['multipath -l', 'multipath -v4 -ll']'
Traceback (most recent call last):
  File "/usr/sbin/sosreport", line 25, in <module>
    main(sys.argv[1:])
  File "/usr/lib/python2.7/site-packages/sos/sosreport.py", line 1389, in main
    sos.execute()
AttributeError: 'list' object has no attribute 'split'

> /usr/lib/python2.7/site-packages/sos/plugins/__init__.py(563)get_cmd_output_now()
-> % (exe.split()[0], time() - start))
(Pdb) bt
  /usr/sbin/sosreport(25)<module>()
-> main(sys.argv[1:])
  /usr/lib/python2.7/site-packages/sos/sosreport.py(1389)main()
-> sos.execute()
  /usr/lib/python2.7/site-packages/sos/sosreport.py(1369)execute()
-> self.collect()
  /usr/lib/python2.7/site-packages/sos/sosreport.py(1144)collect()
-> plug.collect()
  /usr/lib/python2.7/site-packages/sos/plugins/__init__.py(633)collect()
-> self._collect_cmd_output()
  /usr/lib/python2.7/site-packages/sos/plugins/__init__.py(613)_collect_cmd_output()
-> timeout=timeout, runat=runat)
> /usr/lib/python2.7/site-packages/sos/plugins/__init__.py(563)get_cmd_output_now()
-> % (exe.split()[0], time() - start))
(Pdb) result
{'status': 126, 'output': u"timeout: failed to run command '[multipath -l,': Permission denied\n"}

Signed-off-by: Bryn M. Reeves <bmr@redhat.com>