behat: use `@when` flag for testing --require

see #453
When specifying a `--version` for a plugin, always install that version.

Previous behavior was to quit if the plugin was already installed.
behat: always invoke `wp` with a command
behat: don't check DB error message, since it's sometimes encoded and sometimes not.
Merge pull request #553 from wp-cli/fix-553

Fix Behat tests
s/blog-wide/site-wide/g
don't mention deprecated --blog option in synopsis
show warning if --url doesn't receive a value
behat: add test for relative 'require:' path in wp-cli.yml
move 'path:' absolutization logic to Configurator
absolutize 'require:'
Merge pull request #539 from wp-cli/absolutize-require

Absolutize 'require:' paths read from wp-cli.yml
harmonize command descriptions
behat: add test for --url global parameter
set $_SERVER['SERVER_PORT']
behat: remove duplicate check in core.feature
behat: install WP with custom URL
append port to $_SERVER['HTTP_HOST']
don't set $_SERVER['REQUEST_URL']

not used by WordPress and probably bogus
behat: set custom port only in that specific test, since multisite installs don't like it