Allow to generate parent pom reference without relativePath

[monnetchr]

See the discussion https://gitter.im/spring-io/initializr?at=61ded581526fb77b3176a42b

How to reference a parent pom without <relativePath/> ?
<relativePath/> is always added, even when referencing a custom parent pom as documented https://docs.spring.io/initializr/docs/0.11.1/reference/html/#howto-configure-custom-parent-pom
And the code that adds that line is in a private method https://github.com/spring-io/initializr/blob/v0.11.1/initializr-generator/src/main/java/io/spring/initializr/generator/buildsystem/maven/MavenBuildWriter.java#L123