Add option to override default IBM i hostname

[mx-psi]

What does this PR do?

Add hostname field to instance configuration; it allows users to override the default IBM i hostname for a given machine.

Motivation

Customer request.

Additional Notes

OpenAPI supports the hostname format, but

• when validation fails the message just prints a terribly long regex and
• the generated type annotation causes Flake8 raises "syntax error in forward annotation" on regex constraints pydantic/pydantic#2872,

so I decided to go with a simple length check.

Review checklist (to be filled by reviewers)

• Feature or bugfix MUST have appropriate tests (unit, integration, e2e)
• PR title must be written as a CHANGELOG entry (see why)
• Files changes must correspond to the primary purpose of the PR as described in the title (small unrelated changes should have their own PR)
• PR must have changelog/ and integration/ labels attached

[codecov]

Codecov Report

Merging #10383 (67eb80f) into master (d22a79e) will increase coverage by 0.00%.
The diff coverage is 100.00%.

Flag	Coverage Δ
ibm_i	80.85% <100.00%> (+0.80%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

[KSerrania]

LGTM