Add address as an argument for QA::Flow::Login methods

So that we can use the Flow::Login module for Geo tests (which use :geo_primary and :geo_secondary as addresses, instead of :gitlab), change the sign_in and while_signed_in methods to include address as an argument with a default value of :gitlab.

def sign_in(as: nil, address: :gitlab)
  Runtime::Browser.visit(address, Page::Main::Login)
...

Thoughts? cc @gl-quality