[action] [PR:312] [hostcfg] Fix timezone mismatch after image upgrade

[mssonicbld]

Why I did it
To fix an issue where newly installed SONiC images default /etc/localtime to UTC even when the previous image
was configured with and used a different timezone, and config save was executed before the image update.
This causes a mismatch between the timezone defined in CONFIG_DB and the one defined in the Linux file /etc/localtime, because when the new image comes up with a new filesystem, the Linux file /etc/localtime is created with the default UTC, while the timezone value in CONFIG_DB (DEVICE_METADATA|localhost|timezone) is preserved from the previous image.

How I did it
In src/sonic-host-services/scripts/hostcfgd::DeviceMetaCfg:load, we already read the timezone from CONFIG_DB to save it for future events (DeviceMetaCfg:timezone_update observes changes in DEVICE_METADATA).
So at this point, when hostcfgd is up, we can simply update the Linux file /etc/localtime with the initial correct timezone value from CONFIG_DB by executing (only if it differs from the current /etc/localtime):
timedatectl set-timezone
(as is already done in timezone_update).
The same pattern exists in hostcfgd::DnsCfg:load.

How to verify it

1. Set a non-UTC timezone on the current image: config clock timezone Asia/Jerusalem
2. Run config save
3. Install a new image
4. Reboot and check that hostcfgd is up: systemctl status hostcfgd (~1m)
5. Verify that the timezone is identical in CONFIG_DB and /etc/localtime:
   timedatectl
   date
   cat /etc/localtime
   cat /etc/sonic/config_db.json | grep timezone
   hget "DEVICE_METADATA|localhost" "timezone"

[mssonicbld]

Original PR: #312