Add signal handler to change debug level. (sonic-net#96)

* 	[sonic_ax_impl/__main__.py]: Add signal handler to change debug level.
    Today, We can not alter the log level of already running snmp-subagent.
    Adding a signal handler for signal SIGUSR1 to switch log level for both
    sonic_ax_impl and ax_interface logger.
* [README.md]: Updated the doc for snmp_subagent_signal_handler.

README.md

@@ -22,3 +22,17 @@ To run the daemon:
 $ python3.5 -m sonic_ax_impl [-d 10]
 ```
 
+
+To switch log level of already running snmp-subagent process
+
+1.) Find PID of the process.
+
+```
+root 42 1 12 06:37 ? 01:23:46 python3.6 -m sonic_ax_impl
+```
+
+2.) Send SIGUSR1 signal to Process
+```
+root@lnos-x1-a-csw04:/# kill -SIGUSR1 42
+```
+Sending SIGUSR1 signal to process again will reset the log level. 

src/sonic_ax_impl/__main__.py

@@ -1,3 +1,4 @@
+from signal import signal, SIGUSR1
 import logging.handlers
 import os
 import shutil
@@ -11,6 +12,21 @@
 
 LOG_FORMAT = "snmp-subagent [%(name)s] %(levelname)s: %(message)s"
 
+# set signal handlers to switch log level
+def signal_handler_sigusr1(signal, frame):
+    # set log level to debug or revert if set already
+    if sonic_ax_impl.logger.getEffectiveLevel() != logging.DEBUG:
+        sonic_ax_impl.logger.info("signal_handler_sigusr1(): Setting logger level to debug")
+        sonic_ax_impl.logger.setLevel(logging.DEBUG)
+        ax_interface.logger.setLevel(logging.DEBUG)
+    else:
+        sonic_ax_impl.logger.info("signal_handler_sigusr1(): Revert logger level to info")
+        sonic_ax_impl.logger.setLevel(logging.INFO)
+        ax_interface.logger.setLevel(logging.INFO)
+    return
+
+signal(SIGUSR1, signal_handler_sigusr1)
+# end signal handlers
 
 def install_file(src_filename, dest_dir, executable=False):
     dest_file = shutil.copy(src_filename, dest_dir)