loadtest: remove hard-coded config defaults from deployment related code

Description

We should remove hard-coded config from deployment code and instead rely on the configuration files available through the provided GetConfig() methods. Optionally we could also accept them as argument to the `ltctl deployment` command.

Examples where this happens:

https://github.com/mattermost/mattermost-load-test-ng/blob/b8b58286e858a5d09cef7cef9736311339083bef/deployment/terraform/agent.go#L16-L42

https://github.com/mattermost/mattermost-load-test-ng/blob/b8b58286e858a5d09cef7cef9736311339083bef/deployment/terraform/coordinator.go#L51-L68

QA Test Steps

No testing required.

Mana

None

Assignee

Claudio Costa

QA Assignee

Lindy Isherwood

Reporter

Claudio Costa

Epic Link

Fix versions

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Mattermost Team

Platform

Sprint

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GitHub Issue

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