Check for fast SSH lookup when using NFS

Adding a new check

This checks checks if a customer is NFS mounting their git home directory and if so recommends using fast ssh lookup.

Verification steps for review

1. Run check on plain omnibus (should pass)
2. Create NFS mounted disk at /var/opt/gitlab
3. Run check without setting up fast lookup (should fail)
4. Run check after setting up fast lookup via sshd config (should pass)

Author checklist

• After opening the MR:
  • Set it to the current milestone
  • Ask the Maintainer from the Reviewer roulette suggestion for review

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Reviewer checklist

• I followed the verification steps and confirm the functionality of the new check
  • I executed the check as presented in this MR by running the generated playbook with spot
  • In case of unexpected/odd behavior here, verify the generated playbook to account for potential YAML parsing issues
• This check does only perform read operations
• This check does not output more than necessary on stdout for the check to function
• The message explains what it means when this check does not pass
• The workaround_url provides actionable information/steps for affected users
  • Consider if a Knowledge Base article should exist to serve as the ideal workaround URL
• This check is not using the Rails console/runner, or has Maintainer approval for doing so
• If this is a breaking change check:
  • It has the corresponding xx_breaking_changes tag (xx being the major release version for the change)
  • The workaround_url goes to the entry on the https://docs.gitlab.com/update/deprecations/ page
  • The ref_url goes to the deprecation issue linked from that entry
  • The title is the same as that entry
  • The version_started is equal to the announcement_milestone of the deprecation
  • The version_fixed is equal to the removal_milestone of the deprecation