selenium: Clean up wdio.conf.js

selenium: Clean up wdio.conf.js

* Remove redundant user/pass for Jenkins, which is unused now
  that Jenkins jobs set it through ENV instead.
  See also T188520 and Ibb388741fee9 (integration/config).

* Use simple default operator (||) instead of elaborate
  undefined check.

* Document that the inline user/password defaults relate to
  MediaWiki vagrant.

* Set 'logPath' for Jenkins based on LOG_PATH env (from mw-set-env.sh),
  instead of assuming where it will be relative to mediawiki-core.

* Put new lines between blocks and functions.

* Document --no-sandbox hack from 4ff5652501b6cd.

Change-Id: I4c7856d260ded8c94e991a27863b80c9eea2e33e