Add missed docs, use class vars for log conf/context/def.format

Both for syslog and local logging, user can specify custom
name for logging config erb template to be used as well as
custom logging context and default format headers

Change-Id: I8db5bcf419350520db436d16e0ced18296b83c34
Signed-off-by: Bogdan Dobrelya <bogdando@mail.ru>