Correct group file handling when adding users

[tucked]

Fix #63

Before:

# tail foo.group
knox:x:1033:
ambari-server:x:1034:
druid:x:1035:
keyadmin:x:1036:
rangerlookup:x:1037:
yarn-ats:x:1038:
gpadmin:x:1039:
anonymous:x:1040:
admin:x:1041:
hadoopqa:x:1017: hadoop:x:1042:hdfs,mapred,yarn,yarn-ats-hbase,hbase,storm,falcon,tracer,tez,hive,hcat,oozie,zookeeper,ambari-qa,flume,hue,accumulo,hadoopqa,sqoop,spark,mahout,ranger,kms,atlas,ams,kafka,zeppelin,livy,logsearch,infra-solr,activity_analyzer,activity_explorer,HTTP,knox,ambari-server,druid,keyadmin,rangerlookup,yarn-ats,gpadmin,

• The last line is malformed.

  # diff foo.group.bak foo.group
  19d18
  < hadoopqa:x:1017:
  44c43
  < hadoop:x:1042:
  ---
  > hadoopqa:x:1017: hadoop:x:1042:hdfs,mapred,yarn,yarn-ats-hbase,hbase,storm,falcon,tracer,tez,hive,hcat,oozie,zookeeper,ambari-qa,flume,hue,accumulo,hadoopqa,sqoop,spark,mahout,ranger,kms,atlas,ams,kafka,zeppelin,livy,logsearch,infra-solr,activity_analyzer,activity_explorer,HTTP,knox,ambari-server,druid,keyadmin,rangerlookup,yarn-ats,gpadmin,

• Two lines are combined when users are added.

  ---

  After:

  # tail foo.group
  knox:x:1033:
  ambari-server:x:1034:
  druid:x:1035:
  keyadmin:x:1036:
  rangerlookup:x:1037:
  yarn-ats:x:1038:
  gpadmin:x:1039:
  anonymous:x:1040:
  admin:x:1041:
  hadoop:x:1042:hdfs,mapred,yarn,yarn-ats-hbase,hbase,storm,falcon,tracer,tez,hive,hcat,oozie,zookeeper,ambari-qa,flume,hue,accumulo,hadoopqa,sqoop,spark,mahout,ranger,kms,atlas,ams,kafka,zeppelin,livy,logsearch,infra-solr,activity_analyzer,activity_explorer,HTTP,knox,ambari-server,druid,keyadmin,rangerlookup,yarn-ats,gpadmin,

  # diff foo.group.bak foo.group
  44c44
  < hadoop:x:1042:
  ---
  > hadoop:x:1042:hdfs,mapred,yarn,yarn-ats-hbase,hbase,storm,falcon,tracer,tez,hive,hcat,oozie,zookeeper,ambari-qa,flume,hue,accumulo,hadoopqa,sqoop,spark,mahout,ranger,kms, atlas,ams,kafka,zeppelin,livy,logsearch,infra-solr,activity_analyzer,activity_explorer,HTTP,knox,ambari-server,druid,keyadmin,rangerlookup,yarn-ats,gpadmin,