Implement support of log.mode. (#3122)

There was one oversight of #2781 - to influence external tools that Patroni could execute, we set global `umask` value based on permissions of the $PGDATA directory. As a result, it also influenced permissions of log files created by Patroni.

To address the problem we implement two measures:
1. Make `log.mode` configurable.
2. If the value is not set - calculate permissions from the original value of the umask setting.

docs/ENVIRONMENT.rst

@@ -28,6 +28,7 @@ Log
 -  **PATRONI\_LOG\_STATIC\_FIELDS**: add additional fields to the log. This option is only available when the log type is set to **json**. Example ``PATRONI_LOG_STATIC_FIELDS="{app: patroni}"``
 -  **PATRONI\_LOG\_MAX\_QUEUE\_SIZE**: Patroni is using two-step logging. Log records are written into the in-memory queue and there is a separate thread which pulls them from the queue and writes to stderr or file. The maximum size of the internal queue is limited by default by **1000** records, which is enough to keep logs for the past 1h20m.
 -  **PATRONI\_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\_LOG\_MODE**: Permissions for log files (for example, ``0644``). If not specified, permissions will be set based on the current umask value.
 -  **PATRONI\_LOG\_FILE\_NUM**: The number of application logs to retain.
 -  **PATRONI\_LOG\_FILE\_SIZE**: Size of patroni.log file (in bytes) that triggers a log rolling.
 -  **PATRONI\_LOG\_LOGGERS**: Redefine logging level per python module. Example ``PATRONI_LOG_LOGGERS="{patroni.postmaster: WARNING, urllib3: DEBUG}"``

docs/yaml_configuration.rst

@@ -29,6 +29,7 @@ Log
 -  **static_fields**: add additional fields to the log. This option is only available when the log type is set to **json**.
 -  **max\_queue\_size**: Patroni is using two-step logging. Log records are written into the in-memory queue and there is a separate thread which pulls them from the queue and writes to stderr or file. The maximum size of the internal queue is limited by default by **1000** records, which is enough to keep logs for the past 1h20m.
 -  **dir**: Directory to write application logs to. The directory must exist and be writable by the user executing Patroni. If you set this value, the application will retain 4 25MB logs by default. You can tune those retention values with `file_num` and `file_size` (see below).
+-  **mode**: Permissions for log files (for example, ``0644``). If not specified, permissions will be set based on the current umask value.
 -  **file\_num**: The number of application logs to retain.
 -  **file\_size**: Size of patroni.log file (in bytes) that triggers a log rolling.
 -  **loggers**: This section allows redefining logging level per python module