log: modify file log handlers -> use default context

FileLogHandler, TailLogHandler and HeadLogHandler now use
qi::log::context(), not fileLogContext

Change-Id: Ic156c29d72aab766561eac548dca266f57a95607
Reviewed-on: http://gerrit.aldebaran.lan/24196
Tested-by: gerrit
Reviewed-by: pagoya <pagoya@aldebaran-robotics.com>