Remove the hardcoded concurrency limit for ostestr

ostestr by default uses concurrency = ncpu.

This caused all sorts of things to blow up in Cinder
when one got over 6 cpus.  Since the initial merge a
patch has landed that appears to fix the issues, and
we now run reliably at 8 and even 12 cpus.

This patch removes the hard coded concurrency=6 setting
from tox.ini

Change-Id: I483e53acaf1bb71deeba20d6c81184112cbcb046

tox.ini

@@ -14,12 +14,11 @@ install_command = pip install {opts} {packages}
 deps = -r{toxinidir}/requirements.txt
        -r{toxinidir}/test-requirements.txt
 
-# Note, we set concurrency to 6 here as a default
-# ostestr will take the last entry for occurence
-# so if you want to increase/decrease this you
-# can simply add concurrency arg to your tox
+# By default ostestr will set concurrency
+# to ncpu, to specify something else use
+# the concurrency=<n> option.
 # call ie: 'tox -epy27 -- --concurrency=4'
-commands = ostestr --concurrency=6 {posargs}
+commands = ostestr {posargs}
 
 whitelist_externals = bash
 passenv = http_proxy HTTP_PROXY https_proxy HTTPS_PROXY no_proxy NO_PROXY