Some dependency hygiene and update to Boot 1.5.10

The web module doesn't really need to depend on tomcat and all of
the Spring Boot web stack, but users need a way to grab that stuff
quickly if they want it (hence the new starter).

Also removed all spring-boot-starter dependencies from core and
context modules.

spring-cloud-function-samples/function-sample-azure/pom.xml

@@ -34,7 +34,7 @@
 		</dependency>
 		<dependency>
 			<groupId>org.springframework.cloud</groupId>
-			<artifactId>spring-cloud-function-web</artifactId>
+			<artifactId>spring-cloud-starter-function-web</artifactId>
 			<scope>provided</scope>
 		</dependency>
 		<dependency>