replace server urls on tftp pull

[jmozd]

What does this PR change?

The tftp proxy pulls system profiles for PXE boot from the central Uyuni server, where Cobbler generates these pointing typically to the central server's host for http-fetching profiles and installable. When pointing the client to a proxy for installation, these requests should go to the proxy's host instead.
This change adds code in the containerized tftp gateway to replace the according host entries in URLs for grub (grub/system/*) and pxelinux (pxelinux.cfg/01-*) PXE profiles.

GUI diff

No difference.

• DONE

Documentation

• No documentation needed: only internal and user invisible changes

• DONE

Test coverage

• No tests: unsure if the test automatism does handle the test case

Links

https://bugzilla.suse.com/show_bug.cgi?id=1223855

• DONE

Changelogs

Make sure the changelogs entries you are adding are compliant with https://github.com/uyuni-project/uyuni/wiki/Contributing#changelogs and https://github.com/uyuni-project/uyuni/wiki/Contributing#uyuni-projectuyuni-repository

If you don't need a changelog check, please mark this checkbox:

• No changelog needed

If you uncheck the checkbox after the PR is created, you will need to re-run changelog_test (see below)

Re-run a test

If you need to re-run a test, please mark the related checkbox, it will be unchecked automatically once it has re-run:

• Re-run test "changelog_test"
• Re-run test "backend_unittests_pgsql"
• Re-run test "java_pgsql_tests"
• Re-run test "schema_migration_test_pgsql"
• Re-run test "susemanager_unittests"
• Re-run test "javascript_lint"
• Re-run test "spacecmd_unittests"

Before you merge

Check How to branch and merge properly!

[aaannz]

LGTM, but changelog needs to be updated with bsc reference.

[avshiliaev]

Issue:

• https://github.com/SUSE/spacewalk/issues/24288

[srbarrios]

This PR brokes the uyuni-proxy-tftpd service:

May 16 09:32:14 suma-head-pxy uyuni-proxy-tftpd[15283]:     elif path.startswith("pxelinux.cfg/default"):
May 16 09:32:14 suma-head-pxy uyuni-proxy-tftpd[15283]:        ^
May 16 09:32:14 suma-head-pxy uyuni-proxy-tftpd[15283]: SyntaxError: invalid syntax