[aaa] Fix common-auth-sonic.j2 template issue

[ramachandrareddygaddam]

- What I did
To select AAA authentication as local and tacacs+ executed the below CLI command.
"config aaa authentication login local tacacs+"
After executing the command, the common-auth-sonic file is not updated with list of authentication modules.
Also with the above selection, hostcfgd enforcer is closed due to UndefinedError exception.
The crash logs are shown below.

Feb 24 10:34:41.563298 sonic INFO hostcfgd[1382]:   File "/usr/bin/hostcfgd", line 155, in aaa_handler
Feb 24 10:34:41.564132 sonic INFO hostcfgd[1382]:     self.aaacfg.aaa_update(key, data)
Feb 24 10:34:41.564872 sonic INFO hostcfgd[1382]:   File "/usr/bin/hostcfgd", line 78, in aaa_update
Feb 24 10:34:41.565641 sonic INFO hostcfgd[1382]:     self.modify_conf_file()
Feb 24 10:34:41.566395 sonic INFO hostcfgd[1382]:   File "/usr/bin/hostcfgd", line 115, in modify_conf_file
Feb 24 10:34:41.567131 sonic INFO hostcfgd[1382]:     pam_conf = template.render(auth=auth, servers=servers_conf)
Feb 24 10:34:41.567969 sonic INFO hostcfgd[1382]:   File "/usr/local/lib/python2.7/dist-packages/jinja2/environment.py", line 1008, in render
Feb 24 10:34:41.568747 sonic INFO hostcfgd[1382]:     return self.environment.handle_exception(exc_info, True)
Feb 24 10:34:41.569478 sonic INFO hostcfgd[1382]:   File "/usr/local/lib/python2.7/dist-packages/jinja2/environment.py", line 780, in handle_exception
Feb 24 10:34:41.570227 sonic INFO hostcfgd[1382]:     reraise(exc_type, exc_value, tb)
Feb 24 10:34:41.570945 sonic INFO hostcfgd[1382]:   File "/usr/share/sonic/templates/common-auth-sonic.j2", line 22, in top-level template code
Feb 24 10:34:41.571683 sonic INFO hostcfgd[1382]:     auth#011[success=1 default=ignore]#011pam_tacplus.so server={{ last_server.ip }}:{{ last_server.tcp_port }} secret={{ last_server.passkey }} login={{ last_server.auth_type }} timeout={{ last_server.timeout }} {% if server.vrf %} vrf={{ last_server.vrf }} {% endif %} try_first_pass
Feb 24 10:34:41.572397 sonic INFO hostcfgd[1382]:   File "/usr/local/lib/python2.7/dist-packages/jinja2/environment.py", line 430, in getattr
Feb 24 10:34:41.573128 sonic INFO hostcfgd[1382]:     return getattr(obj, attribute)
Feb 24 10:34:41.573860 sonic INFO hostcfgd[1382]: jinja2.exceptions.UndefinedError: 'server' is undefined
Feb 24 10:34:41.580348 sonic NOTICE systemd[1]: hostcfgd.service: Main process exited, code=exited, status=1/FAILURE
Feb 24 10:34:41.582178 sonic NOTICE systemd[1]: hostcfgd.service: Unit entered failed state.
Feb 24 10:34:41.582921 sonic WARNING systemd[1]: hostcfgd.service: Failed with result 'exit-code'.

- How I did it

Fixed the UndefinedError exception. User the last_server as server for VRF configuration.
The variable name server is changed to last_server in the below file
files/image_config/hostcfgd/common-auth-sonic.j2

- How to verify it
Configure AAA login authentication methods as local and tacacs+.
Check the /etc/pam.d/common-auth-sonic is updated correctly.
Check AAA authentication works as expected.

[renukamanavalan]

Looks good. Thanks for catching this bug and fixing it.

[jleveque]

Retest this please