Add OS specific tests to travis

[kreczko]

Since many things can go wrong during setup, we need to test the htcondor installation on each.
I propose to add docker-based unit tests like puppetlabs/puppetlabs-stdlib does for

• Scientific Linux ≥ 6.8
• Scientific Linux ≥ 7.2
• CentOS ≥ 7.2
• Ubuntu 14.04 and 16.04