Add ability to configure app logs to be written to a file (#903)

It gives users the option to send Patroni application logs to a File instead of Standard Out. There are three environment variables that can be set to enable and configure file logging.
1. `PATRONI_FILE_LOG_DIR`: Path to a directory that is writeable by the executing user. Having this variable set is what activates file logging.
2. `PATRONI_FILE_LOG_NUM`: This is a rolling file logger. This variable dictates how many log files are retained.
3. `PATRONI_FILE_LOG_SIZE`: This variable dictates the size at which the logs will roll.

If `PATRONI_FILE_LOG_DIR` is not set than Patroni will log to stderr (default behavior does not change)

Closes https://github.com/zalando/patroni/issues/902

.gitignore

@@ -51,3 +51,6 @@ scm-source.json
 docs/build/
 docs/source/_static/
 docs/source/_templates/
+
+# Pycharm IDE
+.idea/

docs/ENVIRONMENT.rst

@@ -8,11 +8,13 @@ It is possible to override some of the configuration parameters defined in the P
 Global/Universal
 ----------------
 -  **PATRONI\_CONFIGURATION**: it is possible to set the entire configuration for the Patroni via ``PATRONI_CONFIGURATION`` environment variable. In this case any other environment variables will not be considered!
+-  **PATRONI\_FILE\_LOG\_DIR**: Directory to write application logs to. The directory must exist and be writable by the user executing Patroni. If you set this env variable, the application will retain 4 25MB logs by default. You can tune those retention values with PATRONI_LOG_FILE_NUM and PATRONI_LOG_FILE_SIZE (see below).
+-  **PATRONI\_FILE\_LOG\_NUM**: The number of application logs to retain.
+-  **PATRONI\_FILE\_LOG\_SIZE**: Size of log application log file that triggers a log rolling.
 -  **PATRONI\_NAME**: name of the node where the current instance of Patroni is running. Must be unique for the cluster.
 -  **PATRONI\_NAMESPACE**: path within the configuration store where Patroni will keep information about the cluster. Default value: "/service"
 -  **PATRONI\_SCOPE**: cluster name
 -  **PATRONI\_LOGLEVEL**: sets the general logging level (see `the docs for Python logging <https://docs.python.org/3.6/library/logging.html#levels>`_)
--  **PATRONI\_REQUESTS_LOGLEVEL**: sets the logging level for all HTTP requests e.g. Kubernetes API calls (see `the docs for Python logging <https://docs.python.org/3.6/library/logging.html#levels>`_)
 
 Bootstrap configuration
 -----------------------

patroni/__init__.py

@@ -1,3 +1,4 @@
+from logging.handlers import RotatingFileHandler
 import logging
 import os
 import signal
@@ -138,11 +139,22 @@ class Patroni(object):
 
 
 def patroni_main():
+    logdir = os.environ.get('PATRONI_FILE_LOG_DIR', None)
     logformat = os.environ.get('PATRONI_LOGFORMAT', '%(asctime)s %(levelname)s: %(message)s')
     loglevel = os.environ.get('PATRONI_LOGLEVEL', 'INFO')
-    requests_loglevel = os.environ.get('PATRONI_REQUESTS_LOGLEVEL', 'WARNING')
-    logging.basicConfig(format=logformat, level=loglevel)
-    logging.getLogger('requests').setLevel(requests_loglevel)
+
+    if not logdir:
+        logging.basicConfig(format=logformat, level=loglevel)
+    else:
+        logsize = os.environ.get('PATRONI_FILE_LOG_SIZE', 25000000)
+        lognum = os.environ.get('PATRONI_FILE_LOG_NUM', 4)
+
+        root_logger = logging.getLogger()
+        root_logger.setLevel(loglevel)
+        handler = RotatingFileHandler(os.path.join(logdir, 'patroni.log'), mode='a', maxBytes=logsize,
+                                      backupCount=lognum)
+        handler.setFormatter(logging.Formatter(logformat))
+        root_logger.addHandler(handler)
 
     patroni = Patroni()
     try:
@@ -151,6 +163,7 @@ def patroni_main():
         pass
     finally:
         patroni.shutdown()
+        logging.shutdown()
 
 
 def pg_ctl_start(args):