Add login mattermost to Test::Integration::Mattermost

What does this MR do?

• Adds a positional parameter to specify location of the mattermost server to Test::Integration::Mattermost
• Adds some structure for Runtime::Scenario properties
• Adds some modelling of Mattermost pages

All in order to:

• Test logging into Mattermost using oauth against the sibling gitlab instance

Part of gitlab-qa#26 (closed)

Are there points in the code the reviewer needs to double check?

Why was this MR needed?

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests added for this feature/bug
• Review
  • Has been reviewed by UX
  • Has been reviewed by Frontend
  • Has been reviewed by Backend
  • Has been reviewed by Database
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together

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