Added chassis_db_init exeption in get_pmon_daemons func (sonic-net#3626)

What is the motivation for this PR?
After sonic-net/sonic-buildimage#7596 have been merged test_watchdog_reboot started failing on check_pmon_daemons.

How did you do it?
Excluded chassis_db_init from checking as it is executed only one time and exiting.

How did you verify/test it?
Run watchdog_reboot. TC passed

Any platform specific information?
SONiC Software Version: SONiC.master.17861-dirty-20210602.081854
Distribution: Debian 10.9
Kernel: 4.19.0-12-2-amd64
Build commit: e50b6fc3
Build date: Wed Jun 2 13:33:37 UTC 2021

Platform: x86_64-arista_7170_64c
HwSKU: Arista-7170-64C
ASIC: barefoot

Signed-off-by: Roman Savchuk <romanx.savchuk@intel.com>

tests/common/devices/sonic.py

@@ -516,7 +516,7 @@ def get_pmon_daemon_states(self):
         @return: dictionary of { service_name1 : state1, ... ... }
         """
         # some services are meant to have a short life span or not part of the daemons
-        exemptions = ['lm-sensors', 'start.sh', 'rsyslogd', 'start', 'dependent-startup']
+        exemptions = ['lm-sensors', 'start.sh', 'rsyslogd', 'start', 'dependent-startup', 'chassis_db_init']
 
         daemons = self.shell('docker exec pmon supervisorctl status', module_ignore_errors=True)['stdout_lines']