Reduce verbosity for standard case

[Vlatombe]

On every startup I'm getting log lines whereas the migrator does nothing
(because there is nothing to migrate)

[AnneTheAgile]

+1 This is very important.
I see it is up for adoption :( so then how does a PR get added?

This PR is great since it makes logger.finest for entry of the method, and it is already logger.finest when: "Configuration is already up to date for "%s", skipping migration"

Apparently the migrator migrates "from the 1.8 format to the 2.0 format. It does so by removing the SlackJobProperty from the job properties (if there is one)."
However, now being on v2.2 since 8a4fa61 on Mar 11, 2017 CHANGELOG for #294 , we should not be needing this.

Sep 16, 2018 12:23:55 PM jenkins.plugins.slack.config.AbstractProjectConfigMigrator migrate
INFO: Migrating project "_myjob" with type hudson.model.FreeStyleProject

^^ example of a log message, and there is one line for each job, including multi branch repos, one per branch.

[AnneTheAgile]

lgtm

[AnneTheAgile]

Not sure if it worked to LGTM but gave it a go!
//got ok; Your review was submitted successfully.

[AnneTheAgile]

per #320 new maintainers are @kmadel and @gurumaia I hope they will be able to take a look.