This reverts #1899 and #2163, two patches that modified `spark-ec2` so that clusters are identified using tags instead of security groups.  The original motivation for this patch was to allow multiple clusters to run in the same security group.

Unfortunately, tagging is not atomic with launching instances on EC2, so with this approach we have the possibility of `spark-ec2` launching instances and crashing before they can be tagged, effectively orphaning those instances.  The orphaned instances won't belong to any cluster, so the `spark-ec2` script will be unable to clean them up.

Since this feature may still be worth supporting, there are several alternative approaches that we might consider, including detecting orphaned instances and logging warnings, or maybe using another mechanism to group instances into clusters.  For the 1.1.0 release, though, I propose that we just revert this patch.

Author: Josh Rosen <joshrosen@apache.org>

Closes #2225 from JoshRosen/revert-ec2-cluster-naming and squashes the following commits:

0c18e86 [Josh Rosen] Revert "SPARK-2333 - spark_ec2 script should allow option for existing security group"
c2ca2d4 [Josh Rosen] Revert "Spark-3213 Fixes issue with spark-ec2 not detecting slaves created with "Launch More like this""