[SPARK-4137] [EC2] Don't change working dir on user

This issue was uncovered after [this discussion](https://issues.apache.org/jira/browse/SPARK-3398?focusedCommentId=14187471&page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#comment-14187471).

Don't change the working directory on the user. This breaks relative paths the user may pass in, e.g., for the SSH identity file.

```
./ec2/spark-ec2 -i ../my.pem
```

This patch will preserve the user's current working directory and allow calls like the one above to work.

Author: Nicholas Chammas <[email protected]>

Closes #2988 from nchammas/spark-ec2-cwd and squashes the following commits:

f3850b5 [Nicholas Chammas] pep8 fix
fbc20c7 [Nicholas Chammas] revert to old commenting style
752f958 [Nicholas Chammas] specify deploy.generic path absolutely
bcdf6a5 [Nicholas Chammas] fix typo
77871a2 [Nicholas Chammas] add clarifying comment
ce071fc [Nicholas Chammas] don't change working dir

(cherry picked from commit db45f5a)
Signed-off-by: Shivaram Venkataraman <[email protected]>

Author: nchammas

ec2/spark-ec2

@@ -18,5 +18,9 @@
 # limitations under the License.
 #
 
-cd "`dirname $0`"
-PYTHONPATH="./third_party/boto-2.4.1.zip/boto-2.4.1:$PYTHONPATH" python ./spark_ec2.py "$@"
+# Preserve the user's CWD so that relative paths are passed correctly to 
+#+ the underlying Python script.
+SPARK_EC2_DIR="$(dirname $0)"
+
+PYTHONPATH="${SPARK_EC2_DIR}/third_party/boto-2.4.1.zip/boto-2.4.1:$PYTHONPATH" \
+    python "${SPARK_EC2_DIR}/spark_ec2.py" "$@"

ec2/spark_ec2.py

@@ -40,6 +40,7 @@
 from boto import ec2
 
 DEFAULT_SPARK_VERSION = "1.1.0"
+SPARK_EC2_DIR = os.path.dirname(os.path.realpath(__file__))
 
 MESOS_SPARK_EC2_BRANCH = "v4"
 # A URL prefix from which to fetch AMI information
@@ -593,7 +594,14 @@ def setup_cluster(conn, master_nodes, slave_nodes, opts, deploy_ssh_key):
     )
 
     print "Deploying files to master..."
-    deploy_files(conn, "deploy.generic", opts, master_nodes, slave_nodes, modules)
+    deploy_files(
+        conn=conn,
+        root_dir=SPARK_EC2_DIR + "/" + "deploy.generic",
+        opts=opts,
+        master_nodes=master_nodes,
+        slave_nodes=slave_nodes,
+        modules=modules
+    )
 
     print "Running setup on master..."
     setup_spark_cluster(master, opts)
@@ -730,6 +738,8 @@ def get_num_disks(instance_type):
 # cluster (e.g. lists of masters and slaves). Files are only deployed to
 # the first master instance in the cluster, and we expect the setup
 # script to be run on that instance to copy them to other nodes.
+#
+# root_dir should be an absolute path to the directory with the files we want to deploy.
 def deploy_files(conn, root_dir, opts, master_nodes, slave_nodes, modules):
     active_master = master_nodes[0].public_dns_name