[#665,!460] Initial implementation, kea-dhcp4 only

[#665,!460] Initial implementation, kea-dhcp4 only

Added an optional "pattern" parameter to logger output options. It supports
any combination of content described here:

https://log4cplus.sourceforge.io/docs/html/classlog4cplus_1_1PatternLayout.html

src/lib/log/output_option.*
    Added constants to define default patterns:
        OutputOption::DEFAULT_CONSOLE_PATTERN = "%D{%Y-%m-%d %H:%M:%S.%q} %-5p [%c/%i] %m\n";
        OutputOption::DEFAULT_FILE_PATTERN = "%D{%Y-%m-%d %H:%M:%S.%q} %-5p [%c/%i] %m\n";
        OutputOption::DEFAULT_SYSLOG_PATTERN = "%-5p [%c] %m\n";

    OutputOption::pattern_ - new member for conveying pattern string

src/lib/log/logger_manager_impl.*
    LoggerManagerImpl::createConsoleAppender()
    LoggerManagerImpl::createFileAppender()
    LoggerManagerImpl::createSyslogAppender() uses either
    paramater, option.pattern, or the appropriate default pattern

    LoggerManagerImpl::setAppenderLayout() - new, common method to
    set the layout, accepts a pattern string parameter

    LoggerManagerImpl::setConsoleAppenderLayout()
    LoggerManagerImpl::setSyslogAppenderLayout() - deleted

src/lib/process/log_parser.cc
    LogConfigParser::parseOutputOptions() - now supports pattern

src/lib/process/logging_info.*
    LoggingDestination::pattern_  - new member for conveying pattern string

src/bin/dhcp4/dhcp4_lexer.ll
src/bin/dhcp4/dhcp4_parser.yy
    Added "pattern" to the parser grammar