The TSC decided to move to devtoolset-7 on rhel7 to allow for C++14
standard code in mantid.

Don' remove old logs to keep build results more-or-less consistent

Don't fail on removal if the pylint logs don't exist

Remove old log files before a new run. If a pull request has left old files around and the check for changes script tells it not to run then Jenkins will use the old files

Pylint claims the msg-template option produces the same output as the
old parseable format but the Jenkins plugin seems to ignore it all.