Structured logging for the KSK submission event

Example of such message in journalctl:

        ...
	"__REALTIME_TIMESTAMP" : "1522777159435398",
	"PRIORITY" : "5",
	"SYSLOG_IDENTIFIER" : "knotd",
	"ZONE" : "flags.",
	"MESSAGE" : "notice: [flags.] DNSSEC, KSK submission, waiting for confirmation",
	"KEY_SUBMISSION" : "20026",

Fixes #551 (closed)

changed milestone to %2.7.0

added knot label

unmarked as a Work In Progress

changed title from WIP: log: initial implem of structured logging, applied for dnssec keys events to Structured logging, applied for dnssec key events

changed the description

added 201 commits

• 5b7c8843...ff015aa6 - 198 commits from branch master
• 0e732748 - log: add optional parameter to structured logging
• 69a873e9 - dnssec: add KEY_SUBMISSION entry with keytag value to key submission log
• 5fb0811d - doc: add structured logging into operation

Compare with previous version

changed title from Structured logging, applied for dnssec key events to Structured logging for the KSK submission event

changed the description

mentioned in commit 7b52b027

merged