Polish contribution

This commit polihes the original Neo4j contribution in several areas.

Rather than providing the packages to scan, this commit rearranges the
`EntityScan` and `EntityScanRegistrar` so that the logic can be shared
for other components. If no package is provided, scanning now defaults to
the "auto-configured" package(s) and a `@NodeEntityScan` annotation
allows to override that.

The configuration has also been updated to detect the driver based on the
`uri` property. If the embedded driver is available we use that by
default. If it is not available, we're trying to connect to a Neo4j
server running on localhost. It is possible to disable the embedded mode
or set the `uri` parameter explicitly to deviate from these defaults.

The sample no longer relies on the embedded driver for licensing reason:
rather it expects an instance running on localhost (like other
data-related samples) and gracefully ignore any connection error. A
README has been added in the sample to further explain the available
options;

Closes gh-5458

spring-boot-samples/spring-boot-sample-data-neo4j/src/main/resources/application.properties

@@ -1,3 +0,0 @@
-spring.data.neo4j.driver=org.neo4j.ogm.drivers.embedded.driver.EmbeddedDriver
-spring.data.neo4j.domain.packages=sample.data.neo4j
-spring.data.neo4j.session.lifetime=prototype