tester: Fix name for Spark master container
tester: Fix name for Spark master container

The Spark master hostname was recently renamed from "spark-ms" to
"spark-master". This commit also makes use of the new feature to `kelda
ssh` given a hostname rather than blueprint ID.

quilt/quilt,LuiseV/quilt,kelda/kelda,NetSys/quilt,quilt/quilt,quilt/quilt,LuiseV/quilt,LuiseV/quilt,kelda/kelda,quilt/quilt,kelda/kelda,NetSys/quilt,LuiseV/quilt,NetSys/quilt,NetSys/quilt,NetSys/quilt,kelda/kelda,LuiseV/quilt