misc(build): disable assembly in job-server module (spark-jobserver#959)

* misc(build): disable assembly in job-server module

Assembly in job-server module without extras doesn't make much sense; that will not allow most jobs to run

* misc(doc): Clarify assembly building in deploy steps of README

Author: velvia

README.md

@@ -582,7 +582,7 @@ See also running on [cluster](doc/cluster.md), [YARN client](doc/yarn.md), on [E
 
 The `server_start.sh` script uses `spark-submit` under the hood and may be passed any of the standard extra arguments from `spark-submit`.
 
-NOTE: by default the assembly jar from `job-server-extras`, which includes support for SQLContext and HiveContext, is used.  If you face issues with all the extra dependencies, consider modifying the install scripts to invoke `sbt job-server/assembly` instead, which doesn't include the extra dependencies.
+NOTE: Under the hood, the deploy scripts generate an assembly jar from the `job-server-extras` project.  Generating assemblies from other projects may not include all the necessary components for job execution.
 
 ### Context per JVM
 

build.sbt

@@ -15,7 +15,7 @@ lazy val akkaApp = Project(id = "akka-app", base = file("akka-app"))
 lazy val jobServer = Project(id = "job-server", base = file("job-server"))
   .settings(commonSettings)
   .settings(revolverSettings)
-  .settings(Assembly.settings)
+  .settings(assembly := null.asInstanceOf[File])
   .settings(
     description := "Spark as a Service: a RESTful job server for Apache Spark",
     libraryDependencies ++= sparkDeps ++ slickDeps ++ cassandraDeps ++ securityDeps ++ coreTestDeps,
@@ -31,7 +31,6 @@ lazy val jobServer = Project(id = "job-server", base = file("job-server"))
     fullClasspath in Compile <<= (fullClasspath in Compile).map { classpath =>
       extraJarPaths ++ classpath
     },
-    test in assembly := {},
     fork in Test := true
   )
   .settings(publishSettings)