Add functionality to specify the MEDIAWIKI_PASSWORD_VARIABLE environment variable that points to the variable that contains the password

This allows us:
- Not to display the password in Jenkins console log.
- To have different users and passwords when the same test needs to run
on multiple sites (for example on commons.wikimedia.org and
commons.wikimedia.beta.wmflabs.org).

We use the same solution in Ruby:
https://github.com/wikimedia/mediawiki-selenium/blob/master/lib/mediawik
i/selenium/hooks.rb#L15

Bug: 58923
Change-Id: I1c0dd0ed38085e6b9cdce267454550cb49438d5c
Merge "Add functionality to specify the MEDIAWIKI_PASSWORD_VARIABLE environment variable that points to the variable that contains the password"