[ZTP] Add __ztp_profile_loaded guard to __discoverOnly()

[gord1306]

Why I did it

__discoverOnly() has no __ztp_profile_loaded guard, unlike the otherwise equivalent __loadZTPProfile(). In ZTPEngine.executeLoop() the discovery loop picks one of the two depending on use_config_db:

883| if (not use_config_db):
884|     self.__loadZTPProfile("discovery")   # guarded, runs once
887| else:
888|     self.__discoverOnly()                # unguarded, runs every iteration

use_config_db comes from the -o / --use-config-db command line flag (ztp-engine.py:940). On that path __discoverOnly() is called on every iteration of the discovery loop (discovery-interval, 10 s by default), and ztp-profile.sh discoverOnly (ztp-profile.sh:232-258) unconditionally runs systemctl restart rsyslog and systemctl restart interfaces-config.

Purpose

Running ztp-engine.py -o therefore restarts networking roughly every 10 seconds for the whole discovery phase, repeatedly killing the DHCP clients and tearing down the in-band interfaces instead of setting them up once. Each interfaces-config restart drops the in-band netdevs, which breaks anything that runs on them.

Guard __discoverOnly() with __ztp_profile_loaded, mirroring __loadZTPProfile().

Possible concerns and why they do not apply

• Does this break periodic re-discovery? No. executeLoop() still restarts networking every restart-ztp-interval (300 s default); that is an independent code path and is untouched.
• Does discovery setup still happen after a forced restart? Yes. __forceRestartDiscovery() resets __ztp_profile_loaded to False, so the setup is redone, exactly as it is for __loadZTPProfile().
• Do any side effects need to be repeated on every iteration? All six side effects of the discoverOnly branch were checked individually and are one-time and persistent: the rsyslog configuration file and systemctl restart rsyslog, the four static ZTP|mode CONFIG_DB fields, the rsyslog exit-hook symlink, dhcp_policy_create, and the interfaces-config restart. Performing them once is sufficient.
• Can the two paths interfere through the shared flag? No. use_config_db is fixed for the lifetime of executeLoop(), decided once from the command line in main(), so the __loadZTPProfile() and __discoverOnly() paths are mutually exclusive.

How to verify it

Run ZTP discovery with -o / --use-config-db and count the networking restarts during the discovery phase:

grep -c "Finished Update interfaces configuration" /var/log/syslog

Before this change interfaces-config is restarted about every 10 seconds. After it, once during discovery setup, then only on the restart-ztp-interval cadence or after a forced restart.

Notes

• No automated test coverage: every case in tests/test_ztp_engine.py exercises the __loadZTPProfile() path, so the existing tests are unaffected but this change is not covered by them. The test suite could not be executed in the environment used here (ztp / swsscommon not importable), so this change was verified by code inspection only.
• This is a latent defect fix. The default path (use_config_db false) is unaffected.