[CI] update gitlab CI

from the generated configuration at
https://rainboard.laas.fr/project/eigenpy/.gitlab-ci.yml

This:
- add the doc-coverage target, to save the generated documentation an
  code-coverage results
- add the format target, to check the code formatting (allow_failure is
  activated here for now), ref https://github.com/gepetto/linters
- set 18.04 as the main platform (for now, this affects only format &
  doc-coverage target)
- allow automatic retries in case of runner failures
- drop 14.04 checks