[SPARK-3584] sbin/slaves doesn't work when we use password authentication for SSH

Author: Kousuke Saruta <sarutak@oss.nttdata.co.jp>

Closes #2444 from sarutak/slaves-scripts-modification and squashes the following commits:

eff7394 [Kousuke Saruta] Improve the description about Cluster Launch Script in docs/spark-standalone.md
7858225 [Kousuke Saruta] Modified sbin/slaves to use the environment variable "SPARK_SSH_FOREGROUND" as a flag
53d7121 [Kousuke Saruta] Merge branch 'master' of git://git.apache.org/spark into slaves-scripts-modification
e570431 [Kousuke Saruta] Added a description for SPARK_SSH_FOREGROUND variable
7120a0c [Kousuke Saruta] Added a description about default host for sbin/slaves
1bba8a9 [Kousuke Saruta] Added SPARK_SSH_FOREGROUND flag to sbin/slaves
88e2f17 [Kousuke Saruta] Merge branch 'master' of git://git.apache.org/spark into slaves-scripts-modification
297e75d [Kousuke Saruta] Modified sbin/slaves not to export HOSTLIST