Option to disable maintenance screens

[sminnee]

The maintenance screens aren't necessary unless you're taking db snapshots prior to release (which isn't used in all configuration) and they are irritating for people used to no-downtime deployments.

Introduce an option CapistranoDeploymentBackend::setUsesMaintenancePage(), that defaults to true but can be set to false. If false, then enableMaintenance() and disableMaintenance() will be a no-op.

Once this setting is in place, people can use the injector configuration to create an allowed_backend that has no maintenance page.

[sminnee]

/cc @ss23 relates to our email discussion with Jean-Fabien.