Fix next-server behavior

[Lennonka]

What changes are you introducing?

Fixing how the next-server DHCP option is determined when it isn't defined

Why are you introducing these changes? (Explanation, links to references, issues, etc.)

#3532 (comment)

Anything else to add? (Considerations, potential downsides, alternative solutions you have explored, etc.)

• Original behavior removed in Foreman 2.5

Checklists

• I am okay with my commits getting squashed when you merge this PR.
• I am familiar with the contributing guidelines.

Please cherry-pick my commits into:

• Foreman 3.13/Katello 4.15
• Foreman 3.12/Katello 4.14 (Satellite 6.16)
• Foreman 3.11/Katello 4.13 (orcharhino 6.11 on EL8 only; orcharhino 7.0 on EL8+EL9)
• Foreman 3.10/Katello 4.12
• Foreman 3.9/Katello 4.11 (Satellite 6.15; orcharhino 6.8/6.9/6.10)
• Foreman 3.8/Katello 4.10
• Foreman 3.7/Katello 4.9 (Satellite 6.14)
• We do not accept PRs for Foreman older than 3.7.

[github-actions]

The PR preview for 42471e5 is available at theforeman-foreman-documentation-preview-pr-3540.surge.sh

The following output files are affected by this PR:

• Provisioning_Hosts/index-foreman-deb.html
• Provisioning_Hosts/index-foreman-el.html
• Provisioning_Hosts/index-katello.html
• Provisioning_Hosts/index-orcharhino.html
• Provisioning_Hosts/index-satellite.html

show diff

show diff as HTML