Update health endpoint sensitive flag in docs
Provide a way to opt-in to endpoint enablement

Update AbstractEndpoint so that the `enable` property is optional and
when it not specified the `endpoints.enabled` property will be used.

This allows users to switch the way that endpoints are enabled. Rather
than opting-out specific endpoint enablement the `endpoints.enabled`
property can be set to `false` and specific endpoints can be opted-in.

Fixes gh-2102
Setup MongoMappingContext setInitialEntitySet

Update MongoDataAutoConfiguration to set the MongoMappingContext
initialEntitySet by scanning for @Document or @Persistent classes
from AutoConfigurationPackages.

Fixes gh-2107
Polish