rsyslog: use provider => 'upstart'

Similarly to Ifb657c7, switch rsyslog to upstart as well, as it fails on
trusty otherwise. Both this and the SSH one can be removed once we
upgrade to a newer version of puppet, as puppet gained in a 2.7
point-release the line:
    defaultfor :operatingsystem => :ubuntu
which we don't seem to have in precise's 2.7, nor do we have support for
newer style upstart overrides.

Change-Id: I688954780d0c642d9033a940d45584203c9c8369