Disable coverage on jenkins by default.

Always generating a coverage report for all packages takes a lot of time
and I believe that most reports are never viewed.

This changes the default to run without coverage.
It can be re-enabled per package in the test buildout with:

   [test-jenkins]
   test-command = ${test-jenkins:test-command-coverage}
Merge pull request #16 from 4teamwork/jone-no-coverage

Disable coverage on jenkins by default.