Zabbix-agent service doesn't restart after a failure automatically

--- Environment ---
MOS: 9.2
Plugin: zabbix_monitoring-2.5-2.5.5-1.noarch.rpm
Zabbix: 2.2.19-1+trusty

--- Description ---
Zabbix-agent service doesn't start after a failure automatically

--- Steps to reproduce ---
1) Deploy an OpenStack environment

2) Log into a slave node

3) Ensure that Zabbix-agent is up and running:
# ps aux | grep zabbix_agent[d]
# service zabbix-agent status

4) Kill Zabbix-agent processes:
# ps aux | grep zabbix_agent[d] | awk '{print $2}' | xargs kill -9

--- Actual results ---
1) There are no Zabbix-agent processes running:
# ps aux | grep zabbix_agent[d]
#

2) The service is stopped:
# service zabbix-agent status
 * zabbix_agentd is not running

--- Expected behavior ---
Zabbix-agent has restarted automatically and it's working now:

# ps aux | grep zabbix_agent[d]
zabbix 351 0.0 0.0 95020 452 ? S Jun19 0:00 /usr/sbin/zabbix_agentd
zabbix 352 0.0 0.0 95072 664 ? S Jun19 0:21 /usr/sbin/zabbix_agentd: collector [idle 1 sec]
zabbix 353 0.2 0.0 95148 1324 ? S Jun19 2:35 /usr/sbin/zabbix_agentd: listener #1 [waiting for connection]
zabbix 354 0.2 0.0 95124 1336 ? S Jun19 2:32 /usr/sbin/zabbix_agentd: listener #2 [processing request]
zabbix 355 0.2 0.0 95184 1316 ? S Jun19 2:40 /usr/sbin/zabbix_agentd: listener #3 [waiting for connection]
zabbix 356 0.2 0.0 95148 1320 ? S Jun19 2:39 /usr/sbin/zabbix_agentd: listener #4 [waiting for connection]
zabbix 357 0.1 0.0 95148 1328 ? S Jun19 2:16 /usr/sbin/zabbix_agentd: listener #5 [waiting for connection]
zabbix 358 0.1 0.0 95148 1320 ? S Jun19 1:57 /usr/sbin/zabbix_agentd: listener #6 [processing request]
zabbix 359 0.1 0.0 95148 1336 ? S Jun19 2:20 /usr/sbin/zabbix_agentd: listener #7 [waiting for connection]
zabbix 360 0.2 0.0 95148 1316 ? S Jun19 2:28 /usr/sbin/zabbix_agentd: listener #8 [waiting for connection]
zabbix 361 0.1 0.0 95148 1316 ? S Jun19 2:13 /usr/sbin/zabbix_agentd: listener #9 [waiting for connection]
zabbix 362 0.1 0.0 95148 1344 ? S Jun19 2:13 /usr/sbin/zabbix_agentd: listener #10 [waiting for connection]
zabbix 363 0.0 0.0 95020 492 ? S Jun19 0:02 /usr/sbin/zabbix_agentd: active checks #1 [idle 1 sec]

# service zabbix-agent status
 * zabbix_agentd is running