Fixed some errors with HDP usage

[chapsuk]

Hi @opsnull , very thanks for the tool!

Testing on HDP3.1.4 and catch errors:

1. https://github.com/opsnull/hadoop_jmx_exporter/commit/bab8ef029ed6c0ab59665b765dd9d477483546e7

   ...
   File "/exporter/hdfs_journalnode.py", line 40, in collect
   self.hadoop_datanode_metrics.update(self.common_metric_collector.get_metrics(beans, self.target))
   AttributeError: 'JournalNodeMetricCollector' object has no attribute 'hadoop_datanode_metrics' 

2. https://github.com/opsnull/hadoop_jmx_exporter/commit/8bf3bbb7822ae38a75ecc11ac15b08c08264f8ca

   File "/exporter/hdfs_journalnode.py", line 129, in get_metrics
   self.hadoop_journalnode_metrics['JournalNode'][key].add_metric(label, beans[i][metric])
   KeyError: u'NumEditLogsSynced'

3. https://github.com/opsnull/hadoop_jmx_exporter/commit/0e1cedea3e4c9f9b38db5f2032e975cc1e0d64b5

   File "/usr/local/lib/python2.7/site-packages/prometheus_client/utils.py", line 9, in floatToGoString 
   d = float(d) ValueError: ('could not convert string to float: DISK', Metric(hadoop_hdfs_datanode_volume_state, Volume infomation in each path and in each mode, gauge, 
   ...
   Sample(
       name='hadoop_hdfs_datanode_volume_state', 
       labels={
           'cluster': 'cluster_name', 
           'state': 'storageType', 
           'version': u'3.1.1.3.1.4.0-315, r58d0fd3d8ce58b10149da3c717c45e5e57a60d14', 
           '_target': u'my.host.com', 
           'path': '/data'
       }, 
       value='DISK',  <---
       timestamp=None, 
       exemplar=None), 
   ...

[opsnull]

@chapsuk Thank you very much!

fix #3