[sonic_py_common] Fix exception in daemon_base.py

[bingwang-ms]

- Why I did it
There is a syntax error (self is missing for log_info and log_warning) in signal_handler in daemon_base.py, which causes an exception

INFO lldp#supervisord: lldpmgrd Traceback (most recent call last):
INFO lldp#supervisord: lldpmgrd   File "/usr/bin/lldpmgrd", line 224, in <module>
INFO lldp#supervisord: lldpmgrd     main()
INFO lldp#supervisord: lldpmgrd   File "/usr/bin/lldpmgrd", line 220, in main
INFO lldp#supervisord: lldpmgrd     lldpmgr.run()
INFO lldp#supervisord: lldpmgrd   File "/usr/bin/lldpmgrd", line 181, in run
INFO lldp#supervisord: lldpmgrd     (state, c) = sel.select(SELECT_TIMEOUT_MS)
INFO lldp#supervisord: lldpmgrd   File "/usr/lib/python2.7/dist-packages/swsscommon/swsscommon.py", line 965, in select
INFO lldp#supervisord: lldpmgrd     return _swsscommon.Select_select(self, timeout)
INFO lldp#supervisord: lldpmgrd   File "/usr/local/lib/python2.7/dist-packages/sonic_py_common/daemon_base.py", line 52, in signal_handler
INFO lldp#supervisord: lldpmgrd     log_info("DaemonBase: Caught SIGTERM - exiting...")
INFO lldp#supervisord: lldpmgrd NameError: global name 'log_info' is not defined

- How I did it
Fix the syntax error.
- How to verify it
Verified by static syntax analysis.
- Which release branch to backport (provide reason below if selected)

• 201811
• 201911
• 202006

- Description for the changelog

No.

- A picture of a cute animal (not mandatory but encouraged)
No.

[jleveque]

Thanks for the fix, @bingwang-ms!