Cluster: ability to prevent slaves from failing over their masters.

This commit, in some parts derived from PR #3041 which is no longer
possible to merge (because the user deleted the original branch),
implements the ability of slaves to have a special configuration
preventing that they try to start a failover when the master is failing.

There are multiple reasons for wanting this, and the feautre was
requested in issue #3021 time ago.

The differences between this patch and the original PR are the
following:

1. The flag is saved/loaded on the nodes configuration.
2. The 'myself' node is now flag-aware, the flag is updated as needed
   when the configuration is changed via CONFIG SET.
3. The flag name uses NOFAILOVER instead of NO_FAILOVER to be consistent
   with existing NOADDR.
4. The redis.conf documentation was rewritten.

Thanks to @deep011 for the original patch.
