PIPE-2315 - Changes behaviour such that process and validate commands respect server hosts

[elliotforbes]

This PR Changes behaviour for the config process and validate commands to respect the --host flag when it's set to a non-default host.

This improves the UX of the changes introduced in the last PR such that customers don't have to also specify --config-api-host.

Checklist

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• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have checked for similar issues and haven't found anything relevant.
• This is not a security issue (which should be reported here: https://circleci.com/security/)
• I have read Contribution Guidelines.

Internal Checklist

• I am requesting a review from my own team as well as the owning team
• I have a plan in place for the monitoring of the changes that I am making (this can include new monitors, logs to be aware of, etc...)

Changes

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• When instantiating the command, use the host option if it's not equal to the default host. This effectively allows server setups to see no behavioural change for these commands.

Validation

• ./build/darwin/amd64/circleci config --host https://k9s.sphereci.com validate .circleci/config.yml --org-slug gh/circleci - goes to correct https://k9s.sphereci.com/api/v2/me/collaborations and https://k9s.sphereci.com/api/v2/compile-config-with-defaults urls for config compilation
• ./build/darwin/amd64/circleci config validate .circleci/config.yml --org-slug gh/circleci - goes to circleci.com and api.circleci.com respectively
• config validation continues to work for private orbs
• config processing continues to work for private orbs