Add m2e configuration in pom.xml for Eclipse developers

For plugins which don't define their own embedded m2e lifecycle execution information, ZooKeeper should add information to the root pom.xml (in the m2e profile) so that developers using Eclipse don't get stuck with a bunch of errors why they try to load ZooKeeper into that IDE.

https://www.eclipse.org/m2e/documentation/m2e-execution-not-covered.html

The purpose of this is to lower the barrier to contribution for Java developers using Eclipse. It has no other impact on the build.

As an Eclipse user, this will make it significantly easier for me to continue contributing patches to ZooKeeper.