move format strings to class var

Change-Id: I6848737d116c34dc3f95b6c50f45fee6c0b2884a
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

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