mw-api-siteinfo: fetch siteinfo from MW API

Query a MediaWiki API for siteinfo and output the 'generator' field ie
'MediaWiki 1.24alpha'.

--list list all available fields, they can be passed as command line
arguments.

Ex:

 $ mw-api-siteinfo.py http://example.com/w/api.php git_hash git_branch
 f2912ce5dd39b17f66d1d894593f29f383e43f41
 master
 $

Will be used for the CI browsertests jobs that need to figure out which
branch is being run on test2/production wikis in order to run the tests
from their branches.

Bug: 62509
Change-Id: If3db8f480ce0d590522741d40fb8e43b753efb41