[SPARK-6552][Deploy][Doc]expose start-slave.sh to user and update outdated doc

https://issues.apache.org/jira/browse/SPARK-6552

/cc srowen

Author: WangTaoTheTonic <wangtao111@huawei.com>

Closes #5205 from WangTaoTheTonic/SPARK-6552 and squashes the following commits:

b02263c [WangTaoTheTonic] use less than rather than less equal
f0fa408 [WangTaoTheTonic] expose start-slave.sh

docs/spark-standalone.md

@@ -24,7 +24,7 @@ the master's web UI, which is [http://localhost:8080](http://localhost:8080) by
 
 Similarly, you can start one or more workers and connect them to the master via:
 
-    ./bin/spark-class org.apache.spark.deploy.worker.Worker spark://IP:PORT
+    ./sbin/start-slave.sh <worker#> <master-spark-URL>
 
 Once you have started a worker, look at the master's web UI ([http://localhost:8080](http://localhost:8080) by default).
 You should see the new node listed there, along with its number of CPUs and memory (minus one gigabyte left for the OS).
@@ -81,6 +81,7 @@ Once you've set up this file, you can launch or stop your cluster with the follo
 
 - `sbin/start-master.sh` - Starts a master instance on the machine the script is executed on.
 - `sbin/start-slaves.sh` - Starts a slave instance on each machine specified in the `conf/slaves` file.
+- `sbin/start-slave.sh` - Starts a slave instance on the machine the script is executed on.
 - `sbin/start-all.sh` - Starts both a master and a number of slaves as described above.
 - `sbin/stop-master.sh` - Stops the master that was started via the `bin/start-master.sh` script.
 - `sbin/stop-slaves.sh` - Stops all slave instances on the machines specified in the `conf/slaves` file.

sbin/start-slave.sh

@@ -17,8 +17,14 @@
 # limitations under the License.
 #
 
-# Usage: start-slave.sh <worker#> <master-spark-URL>
-#   where <master-spark-URL> is like "spark://localhost:7077"
+# Starts a slave on the machine this script is executed on.
+
+usage="Usage: start-slave.sh <worker#> <spark-master-URL> where <spark-master-URL> is like "spark://localhost:7077"
+
+if [ $# -lt 2 ]; then
+  echo $usage
+  exit 1
+fi
 
 sbin="`dirname "$0"`"
 sbin="`cd "$sbin"; pwd`"

sbin/start-slaves.sh

@@ -17,6 +17,8 @@
 # limitations under the License.
 #
 
+# Starts a slave instance on each machine specified in the conf/slaves file.
+
 sbin="`dirname "$0"`"
 sbin="`cd "$sbin"; pwd`"