[2018.3] Allow running runtests.py using tox

[s0undt3ch]

What does this PR do?

[2018.3] Allow running runtests.py using tox

[codecov]

Codecov Report

Merging #51313 into 2018.3 will increase coverage by 1.12%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           2018.3   #51313      +/-   ##
==========================================
+ Coverage   32.29%   33.42%   +1.12%     
==========================================
  Files        1469     1469              
  Lines      242239   241914     -325     
  Branches    51531    50159    -1372     
==========================================
+ Hits        78231    80848    +2617     
+ Misses     155363   152140    -3223     
- Partials     8645     8926     +281

Flag	Coverage Δ
#centos7	33.42% <ø> (?)
#debian8	?
#py2	33.42% <ø> (?)
#py3	?
#ubuntu1804	?

Impacted Files	Coverage Δ
salt/transport/frame.py	27.27% <0%> (-36.37%)	⬇️
salt/auth/ldap.py	9.8% <0%> (-20.79%)	⬇️
salt/pillar/varstack_pillar.py	72.22% <0%> (-16.67%)	⬇️
salt/utils/saltclass.py	65.26% <0%> (-12.58%)	⬇️
salt/utils/versions.py	61.87% <0%> (-12.24%)	⬇️
salt/modules/mod_random.py	51.56% <0%> (-10.94%)	⬇️
salt/modules/kmod.py	43.93% <0%> (-9.85%)	⬇️
salt/auth/pki.py	21.31% <0%> (-9.84%)	⬇️
salt/modules/qemu_img.py	68.18% <0%> (-9.1%)	⬇️
salt/utils/pycrypto.py	39.28% <0%> (-8.93%)	⬇️
... and 234 more

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