Use Docker and Maven Release Plugin to prepare ZooKeeper releases

In 3.5.5 and 3.5.6 we followed a new release procedure based on Maven:

https://cwiki.apache.org/confluence/display/ZOOKEEPER/HowToRelease+using+maven

That procedure needed some "manual" parts to change the project version inside pom files and also inside the sources of the C Client.

We can automate more and more in order to make the release procedure mostly automatic.

We should also use 'docker' in order to have a reproducible build environment, expectially for the 'convenience binaries':

• Java version (we want to build the project with Java 8 in 3.6.0)
• C client (tools, system headers and openssl version)