Retain additional cron history by default

[mpchadwick]

Description

Changes default cron success / failure history to seven days.

There's no reason the clear the history out so fast other than to make it impossible to audit, if / when there is an issue...

Manual testing scenarios

1. In the admin panel go to Stores > Configuration > Advanced > System > Cron
2. Validate that "Success History Lifetime" and "Failure History Lifetime" is 10080 for both the default and the index group.

Contribution checklist

• Pull request has a meaningful description of its purpose
• All commits are accompanied by meaningful commit messages
• All new or changed code is covered with unit/integration tests (if applicable)
• All automated tests passed successfully (all builds on Travis CI are green)

[davidalger]

@mpchadwick I wholeheartedly agree with this change. Let's see if I can't get it merged in! :)

[davidalger]

@mpchadwick Would you be able to create PRs to apply this change to 2.2-develop and possibly 2.1-develop as well so it can make it into those release lines as well?

[mpchadwick]

@davidalger will submit additional PRs against those branches when I have a moment.