[SPARK-2746] Set SBT_MAVEN_PROFILES only when it is not set explicitly by the user.

Since it's bash, you could do it with a one-liner:

export SBT_MAVEN_PROFILES=${SBT_MAVEN_PROFILES:-"-Pyarn -Phadoop-2.3 -Dhadoop.version=2.3.0"}

But the current version is fine too.