Add devtoolset 8 for el6 and el7 stack matrix jobs
Add devtoolset 8 for el6 and el7 stack matrix jobs

- Add devtoolset8 for el6 to map
- Add el6 and el7 jobs with devtoolset 8 to stack matrix
- Allow those jobs to fail

lsst-sqre/jenkins-dm-jobs,jhoblitt/jenkins-dm-jobs