'mvn install' involves building the binary which makes Travis more failed

[jihoonson]

Currently the install phase consists of compiling the sources, installing the binaries in the local repository, and building the distribution tarball. Here, the building the distribution tarball involves pulling extension libraries (https://github.com/apache/incubator-druid/blob/master/distribution/pom.xml#L51-L141) which are basically included in the distribution tarball. This makes mvn install slower even when we want to just compile the sources. Also, in recent Travis jobs, this is causing frequent job failing because of timeout. For example, in https://travis-ci.org/apache/incubator-druid/builds/414891251, 4 jobs failed due to timeout while building the distribution tarball.

So, I propose to distinguish just compiling the sources (and running unit tests) and building the distribution tarball. Probably we can add an option such as -Dtar like other projects.