Allow override $host,$port with env.{host,port} and Fix values calculation

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Note: quoted from: https://github.com/mickm/elasticsearch-munin/pull/3
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$host and $port variables are hardcoded to 'localhost' and 9200

This commit allow to override these values.

```
# cat /etc/munin/plugin-conf.d/elasticsearch
[elasticsearch_*]
env.host x.x.x.x
env.port 9300
```

Values are wrong when having multi nodes. The last one is used.

```
"cluster_name": "elasticsearch", 
"nodes": {
    "1pZgzWkNTUqJsPfYcxH6qA": {
(...)
            "docs": {
                "count": 1221827, 
                "deleted": 0
            },
(...)
    "XU_9yAt0R-C6vyaK_iRVpw": {
(...)
            "docs": {
                "count": 0, 
                "deleted": 0
            },
```
Plugin return 0 instead of 1221827 + 0 = 1221827

Values in hash %out are not initialize to zero.