Make logging configurable via YAML file (#927)

It allows changing logging settings in runtime by updating config and doing reload or sending `SIGHUP` to the Patroni process.
Important! Environment configuration names related to logging were renamed and documentation accordingly updated. For compatibility reasons Patroni still accepts `PATRONI_LOGLEVEL` and `PATRONI_FORMAT`, but some other variables related to logging, which were introduced only
recently (between releases), will stop working. I think it is ok, since we didn't release the new version yet and therefore it is very unlikely that somebody is using them except authors of corresponding PRs.

Example of log section in the config file:
```yaml
log:
  dir: /where/to/write/patroni/logs  # if not specified, write logs to stderr
  file_size: 50000000  # 50MB
  file_num: 10  # keep history of 10 files
  dateformat: '%Y-%m-%d %H:%M:%S'
  loggers:  # increase log verbosity for etcd.client and urllib3
    etcd.client: DEBUG
    urllib3: DEBUG
```

tests/test_config.py

@@ -1,6 +1,6 @@
 import os
-import unittest
 import sys
+import unittest
 
 from mock import MagicMock, Mock, patch
 from patroni.config import Config
@@ -30,6 +30,8 @@ class TestConfig(unittest.TestCase):
             'PATRONI_NAME': 'postgres0',
             'PATRONI_NAMESPACE': '/patroni/',
             'PATRONI_SCOPE': 'batman2',
+            'PATRONI_LOGLEVEL': 'ERROR',
+            'PATRONI_LOG_LOGGERS': 'patroni.postmaster: WARNING, urllib3: DEBUG',
             'PATRONI_RESTAPI_USERNAME': 'username',
             'PATRONI_RESTAPI_PASSWORD': 'password',
             'PATRONI_RESTAPI_LISTEN': '0.0.0.0:8008',