hostcfgd: Add one shot timer to reload tacacs config from CONFIG-DB

[renukamanavalan]

Why I did it

There is a small window between load & listen to config-DB. If TACACS config got updated during that gap, the listen will not show it, hence hostcfgd would miss it, until another update.

How I did it

porting PR #8223, which uses one shot timer to reload tacacs config.

How to verify it

Copy this binary to a 201811 device and
Run config reload & restart of hostcfd concurrently, multiple times to see, if hostcfgd miss the update or not.

Ran a repeated config reload for testing.
It did succeed.
But it could not really hit the repro, which is kind of expected as it is is very hard to hit the repro in 201811

[Tweaked code with a log message, that dumps the read value with isTimer to indicate normal load vs via timer]

admin@str-s6000-acs-8:/var/log$ zgrep -i istimer syslog*
syslog:Feb 16 18:53:25.006739 str-s6000-acs-8 INFO hostcfgd: pam.d files updated isTimer=False auth={'login': 'tacacs+', 'failthrough': True} global={'auth_type': 'login', 'timeout': '5', 'passkey': 't*****'}
syslog:Feb 16 18:53:55.707662 str-s6000-acs-8 INFO hostcfgd: pam.d files updated isTimer=True auth={'login': 'tacacs+', 'failthrough': True} global={'auth_type': 'login', 'timeout': '5', 'passkey': 't*****'}
syslog.1:Feb 16 18:29:18.661496 str-s6000-acs-8 INFO hostcfgd: pam.d files updated isTimer=False auth={'login': 'tacacs+', 'failthrough': True} global={'auth_type': 'login', 'timeout': '5', 'passkey': 't*****'}
syslog.1:Feb 16 18:29:49.193510 str-s6000-acs-8 INFO hostcfgd: pam.d files updated isTimer=True auth={'login': 'tacacs+', 'failthrough': True} global={'auth_type': 'login', 'timeout': '5', 'passkey': 't*****'}
syslog.1:Feb 16 18:41:22.170889 str-s6000-acs-8 INFO hostcfgd: pam.d files updated isTimer=False auth={'login': 'tacacs+', 'failthrough': True} global={'auth_type': 'login', 'timeout': '5', 'passkey': 't*****'}
...
admin@str-s6000-acs-8:~$ cat scripts/r.sh 
#! /bin/bash

set -x

for ((i=0; i<10; ++i))
do
    echo "-------------------- $i -----------------------"
    sleep 10m
    sudo config reload -y
    t=$(cat /etc/pam.d/common-auth-sonic  | grep testing123 | wc -l) 
    if [ "$t" -eq "1" ]; then
        echo "Success"
    else
        echo "Failure"
        exit -1
    fi
done


admin@str-s6000-acs-8:~$ 

Which release branch to backport (provide reason below if selected)

This is the backport to 201811

• 201811
• 201911
• 202006
• 202012
• 202106
• 202111

Description for the changelog

Add a one shot timer after load; Reload tacacs config from DB, upon the timer.

Link to config_db schema for YANG module changes

A picture of a cute animal (not mandatory but encouraged)