RFR: JDK-8274311: Make build.tools.jigsaw.GenGraphs more configurable

[Mandy Chung]

GenGraphs tool generates the module graph. It currently supports the configuration via javadoc-graphs.properties. However, `make/jdk/src/classes/build/tools/jigsaw/javadoc-graphs.properties` only documents two properties. It should be updated to cover all configurable properties.

There are a couple other properties not configurable such as nodesep and node margin. This extends the configuration to allow to set additional properties. 

This also fixes `requiresMandatedColor` in javadoc-graphs.properties to light gray to match the default configuration in the implementation, i.e. the color of the edge to java.base.  It seems a bug that was unnoticed until Alex and Iris spotted it.

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Commit messages:
 - JDK-8274311: Make build.tools.jigsaw.GenGraphs more configurable

Changes: https://git.openjdk.java.net/jdk/pull/5690/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=5690&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8274311
  Stats: 167 lines in 3 files changed: 100 ins; 33 del; 34 mod
  Patch: https://git.openjdk.java.net/jdk/pull/5690.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/5690/head:pull/5690

PR: https://git.openjdk.java.net/jdk/pull/5690