Allow specifying a level for the nginx error logs

What does this MR do?

Allow specifying a level for the nginx error logs. The default verbosity level is error, so we'll keep using that as a default.

For the current iteration, this setting will affect all nginx server blocks (That is, it is not possible to set the verbosity per server block).

Related issues

Internal Zendesk Ticket: https://gitlab.zendesk.com/agent/tickets/229320

Checklist

See Definition of done.

For anything in this list which will not be completed, please provide a reason in the MR discussion

Required

• Merge Request Title, and Description are up to date, accurate, and descriptive
• MR targeting the appropriate branch
• MR has a green pipeline on GitLab.com
• Pipeline is green on the dev.gitlab.org mirror for this branch if the change is touching anything besides documentation or internal cookbooks
• trigger-package has a green pipeline running against latest commit

Expected (please provide an explanation if not completing)

• Test plan indicating conditions for success has been posted and passes
• Documentation created/updated
• Tests added
• Integration tests added to GitLab QA
• Equivalent MR/issue for the GitLab Chart opened