Prepare for grid

[wiiitek]

Description

Let's introduce a way to use different web driver than default.
This PR introduces changes for test to spoecify preferred browser ID.
This way we could have more than one WebDriver factories active. Usage example:

<test name="S-comparator-Layout" browser="chrome">
    ...

Motivation and Context

It enables us to work gradually on new web drivers (selenium grid).
We are still maintaining current way of usng Firefox 38.6 ESR

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.

---

I hereby agree to the terms of the AET Contributor License Agreement.

[malaskowski]

How about renaming test parameter to browser?
I know the name webDriverId is more appropriate here from the technical perspective, but when we make changes in the AET suite DSL, it should be more user-friendly.