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