maint - fix visibility of Logger::Base#start and log

start and log are parts of the public api of Logger plugins

Logger#start is invoked by MCollective::Log#configure
Logger#log is invoked by MCollective::Log#log

As such these virtual methods should not be considered part of
MCollective::Logger::Base's private api.
add reopen to Logger::Base
Add Log.reopen that just delegates to @logger.reopen
reopen implementation for the FileLogger

we need to make note of the current loglevel, as it may not be the version from
the config file that start will assert