MEDIUM: cfgparse: add KWF_DISCOVERY keyword flag

MEDIUM: cfgparse: add KWF_DISCOVERY keyword flag

This commit is a part of the series to add a support of discovery mode in the
configuration parser and in initialization sequence.

So, let's add here KWF_DISCOVERY flag to distinguish the keywords,
which should be parsed in "discovery" mode and which are needed for master
process, from all others. Keywords, that should be parsed in "discovery" mode
have its dedicated parser funtions. Let's tag these functions with
KWF_DISCOVERY flag in keywords list. Like this, only these keyword parsers
might be called during the first configuration read in discovery mode.