Load full omnibus configuration instead of line by line

What does this MR do and why?

Describe in detail what your merge request does and why.

OmnibusConfiguration is capable of handling multiline configs. This MR passes the entire config string instead of line by line.

Fixes issue seen in https://gitlab.com/gitlab-org/gitlab/-/jobs/5521319445

Also this MR:

• Removes Test::Integration::OAuth scenario and updates Test::Instance::Image to accept hostname and TLS boolean
• Updates the docs for running OAuth tests and new variables

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