[hostcfgd] [202012] Fix the delay type to 'boot' delay instead of a u…

…nit activation delay (#8896)

#### Why I did it
With current code the delay will take place even if simple 'config reload' command executed and this is not desired.
This delay should be used only when fast-rebooting.

#### How I did it
Change the type of delay to OnBootSec instead of OnActiveSec.

#### How to verify it
Fast-reboot with this PR and observe the delay.
Run 'config-reload' command and observe no delay is running.

src/sonic-host-services-data/debian/sonic-host-services-data.hostcfgd.timer

@@ -3,7 +3,8 @@ Description=Delays hostcfgd daemon until SONiC has started
 PartOf=hostcfgd.service
 
 [Timer]
-OnActiveSec=1min 30 sec
+OnUnitActiveSec=0 sec
+OnBootSec=1min 30 sec
 Unit=hostcfgd.service
 
 [Install]