Parallel test for master

[github at samba.org]

New comment by abartlet on Samba Github repository

https://github.com/samba-team/samba/pull/137#issuecomment-370020529
Comment:
Sadly this pushes the samba-o3 branch over the 50min maximum for travis-ci (which is one goal of these patches, to make travis-ci a reasonable check for python3 patches in particular).

We probably need to put 'none' in a different (new?) environment and split up more of the ad_dc tests.

If someone wants to take this on, please do!