Doh, tox is installed by puppet using pip as a provider:

modules/contint/manifests/packages/labs.pp

package { 'tox':
    ensure   => present,
    provider => 'pip',
    require  => Package['python-pip'],
}

Which mean the tox version is whatever was the latest when the instance was created and puppet never attempt to upgrade it. For the Precise instances I end up with:

slave1001 : tox 1.8.1
slave1002 : tox 1.7.0
slave1003 : tox 1.7.1
slave1004 : tox 1.8.0

I am not sure why it would fail with a version of tox and not with another one :-/

In this specific case, because of https://bitbucket.org/hpk42/tox/pull-request/85/fix-command-expansion-and-parsing/diff

Basically, what happened is that the behavior of

variable = a b c
command = nosetests {variable}

changed. Pre-1.7, this meant

nosetests a b c

and this was changed in 1.7 to

nosetests "a b c"

and changed back again in 1.7.2 to

nosetests a b c

So setting versions explicitly in the manifest might be a good idea.

Thanks to have tracked the issue. I remember now I have been hit by that tox regression locally when 1.7 got released but never bothered with it.

I will apply pip install --upgrade tox on all the slaves.