In ce3b81b953f we changed Test::Integration::Mattermost#perform to
consume a required positional parameter and pass on the rest of the
arguments to the superclass. There was an error in implementation here
and instead of splatting the files array to produce no additional
arguments, we passed in the default case an empty array.

This had the effect of calling rspec with no test files,
rather than expected default of 'qa/specs/features`

Here we correct that bug, and re-add the :core suite, as that was a poor
deuction in the investagtion for 74d6b8a211.

In !14818 we added the ability to specify the example groups that should
run via the `-t` (tag) option to rspec.  It was assumed that the
inclusion filter generated `{:core => true, :mattermost => true}` would
run examples tagged :core OR :mattermost, when instead it means :core
AND :mattermost.

Here in order to assure at least some tests run, we specify that
the :mattermost tagged examples are the only ones we care about.

* Adds Runtime::Scenario as a place to record the properties of the
  Scenario we're interested in.

* Adds a positional argument to Test::Integration::Mattermost to specify
  Runtime::Scenario.mattermost

* Adds Page::Mattermost::{Main,Login} to model the mattermost pages we
  need to interact with.

* Adds qa/features/mattermost/login_spec to perform a login via oauth
  and ensure we are logged in.

For added confidence, and because I plan to fiddle with some behaviours
shortly, add spec testing to the newly extracted
QA::Scenario::Entrypoint class.

We can currently reduce the number of times we specify the tag of a
given example group by moving them hoisting them up one level to the
containing `feature`

This was added as part of the previous filtering via RSpec::Config.  It
should have been removed previously as part of moving the logic to
specify the tags we wanted in Specs::Runner

It's adding additional potentially confusing behaviour to derive test
tagging from the file path, as it's not immediately apparent where that
lives when considering the tests in isolation.  Instead, just explicitly
tag.

* Remove the tagging of examples based on :file_path
* Explicitly tags scenarios as :core, or :mattermost

Here we convert Specs::Runner#rspec to use keyword arguments[1] and pass
named parameters rather than a pre-processed array of cli switches.
This allows parameter to cli logic to live just in Specs::Runner.

[1] https://robots.thoughtbot.com/ruby-2-keyword-arguments

Entrypoint is intended as a base class, so mention that in the comment.