Add documentation for how to setup eremetic framework to support failovers

[bbannier]

Currently there is not clear and sound documentation on how users should set up eremetic so that scheduler failovers are supported. Since currently eremetic does not itself persist its framework_info or master-assigned framework_id, users are required to work around this.

Since Mesos does not have explicit support for self-assigned framework_id values, the workaround mentioned in #181 is not safe. Mesos will start enforcing some validation on used framework_id values with MESOS-9469 which might break workflows where users self-assign certain values.

A sound approach would be

1. start the scheduler with no framework_id set in the config file
2. watch logs for the assigned framework_id
3. update config file to contain the master-assigned framework_id
4. reregister scheduler

Eremetic should have some clear documentation on how to set up eremetic in a sound way.

[keis]

Thanks for the heads up @bbannier that does sound like the simplest way of doing this for now. Maybe we could display the frameworkid somewhere to make it easier to find.