Expand environment variables when loading the noxfile from provided path

[s0undt3ch]

Our CI setup uploads the repo to the system's temporary directory, which on Linux is known(or commonly enough /tmp) but on windows, %TEMP% is used:

❯ bundle exec kitchen verify py3-windows-2012r2
-----> Starting Kitchen (v1.23.5)
-----> Verifying <py3-windows-2012r2>...
       [Nox] Verify on instance py3-windows-2012r2 with state={:username=>"administrator", :auto_security_group_id=>"sg-0cf1cc52c2d0e5d24", :server_id=>"i-0f2b90b1cfedb970e", :hostname=>"10.27.2.213", :password=>"uDU9*;ZH(j", :last_action=
>"verify", :last_error=>nil}
       Running Command: nox -f %TEMP%\kitchen/testing/noxfile.py -e "runtests-3(coverage=True)" -- --output-columns=120 --sysinfo --xml=%TEMP%\kitchen/testing/artifacts/xml-unittests-output --names-file=%TEMP%\kitchen\testing\tests\whiteli
st.txt --transport=zeromq -v --run-destructive --ssh-tests    -n tests\unit 2>&1
       nox : Noxfile C:\Users\Administrator\Documents\%TEMP%\kitchen\testing\noxfile.py not found.
       At line:1 char:1
       + nox -f %TEMP%\kitchen/testing/noxfile.py -e "runtests-3(coverage=True)" -- --out ...
       + ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
           + CategoryInfo          : NotSpecified: (Noxfile C:\User...e.py not found.:String) [], RemoteException
           + FullyQualifiedErrorId : NativeCommandError
        
       Verify command failed :: WinRM exited (2) for command: [nox -f %TEMP%\kitchen/testing/noxfile.py -e "runtests-3(coverage=True)" -- --output-columns=120 --sysinfo --xml=%TEMP%\kitchen/testing/artifacts/xml-unittests-output --names-fi
le=%TEMP%\kitchen\testing\tests\whitelist.txt --transport=zeromq -v --run-destructive --ssh-tests    -n tests\unit 2>&1]                                                                                                                      
       Copying %TEMP%\kitchen/testing/artifacts to /home/vampas/projects/SaltStack/salt/features/nox-2017.7/
       Copying c:/salt/var/log/salt/minion to artifacts/logs/minion
>>>>>> Failed to copy c:/salt/var/log/salt/minion to artifacts/logs/minion :: ${e}
       Finished verifying <py3-windows-2012r2> (0m14.00s).
-----> Kitchen is finished. (0m16.95s)

This PR processes the passed noxfile through os.path.expandvars to accommodate the above scenario.

[dhermes]

LGTM, I'll leave it to @theacodes to decide about the coverage

[theacodes]

Thanks for writing this @s0undt3ch and thanks for reviewing, @dhermes. :)

[s0undt3ch]

Anytime!