SPARK-1404: Always upgrade spark-env.sh vars to environment vars

This was broken when spark-env.sh was made idempotent, as the idempotence check is an environment variable, but the spark-env.sh variables may not have been.

Tested in zsh, bash, and sh.

Author: Aaron Davidson <aaron@databricks.com>

Closes #310 from aarondav/SPARK-1404 and squashes the following commits:

c3406a5 [Aaron Davidson] Add extra export in spark-shell
6a0e340 [Aaron Davidson] SPARK-1404: Always upgrade spark-env.sh vars to environment vars

bin/load-spark-env.sh

@@ -30,6 +30,9 @@ if [ -z "$SPARK_ENV_LOADED" ]; then
   use_conf_dir=${SPARK_CONF_DIR:-"$parent_dir/conf"}
 
   if [ -f "${use_conf_dir}/spark-env.sh" ]; then
+    # Promote all variable declarations to environment (exported) variables
+    set -a
     . "${use_conf_dir}/spark-env.sh"
+    set +a
   fi
 fi

bin/spark-shell

@@ -127,7 +127,7 @@ function set_spark_log_conf(){
 
 function set_spark_master(){
     if ! [[ "$1" =~ $ARG_FLAG_PATTERN ]]; then
-        MASTER="$1"
+        export MASTER="$1"
     else
         out_error "wrong format for $2"
     fi
@@ -145,7 +145,7 @@ function resolve_spark_master(){
     fi
 
     if [ -z "$MASTER" ]; then
-        MASTER="$DEFAULT_MASTER"
+        export MASTER="$DEFAULT_MASTER"
     fi
 
 }