Add fork option for mvn spring-boot:run

Update RunMojo to include a `fork` option and change the default
to only fork if agent or jvmArguments are specified.

Fixes gh-1412

spring-boot-tools/spring-boot-maven-plugin/src/it/run-fork/pom.xml

@@ -0,0 +1,31 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
+	xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
+	<modelVersion>4.0.0</modelVersion>
+	<groupId>org.springframework.boot.maven.it</groupId>
+	<artifactId>run-fork</artifactId>
+	<version>0.0.1.BUILD-SNAPSHOT</version>
+	<properties>
+		<project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
+	</properties>
+	<build>
+		<plugins>
+			<plugin>
+				<groupId>@project.groupId@</groupId>
+				<artifactId>@project.artifactId@</artifactId>
+				<version>@project.version@</version>
+				<executions>
+					<execution>
+						<phase>package</phase>
+						<goals>
+							<goal>run</goal>
+						</goals>
+						<configuration>
+							<fork>true</fork>
+						</configuration>
+					</execution>
+				</executions>
+			</plugin>
+		</plugins>
+	</build>
+</project>

spring-boot-tools/spring-boot-maven-plugin/src/it/run-fork/src/main/java/org/test/SampleApplication.java

@@ -0,0 +1,9 @@
+package org.test;
+
+public class SampleApplication {
+
+	public static void main(String[] args) {
+		System.out.println("I haz been run");
+	}
+
+}

spring-boot-tools/spring-boot-maven-plugin/src/it/run-fork/verify.groovy

@@ -0,0 +1,3 @@
+def file = new File(basedir, "build.log")
+return file.text.contains("I haz been run")
+