Whitespace in field names with outputs.graphite

[kostasb]

1.0-beta3

Field names that include a whitespace generate malformed output when using the Graphite format.

Example input:
ameasurement,atag=test a\ field="value"

Output in Line Protocol:
ameasurement,atag=test a\ field="value" 1471350314460772128

Output in Graphite format:
ameasurement.test.a field "value" 1471350314

Graphite messages are of the form:metric_path value timestamp\n
The metric path can not include a whitespace.

This is not an issue with tags because whitespaces in tag values are replaced with a "_" separator.

@sparrc A configurable separator to replace whitespaces in field names for the graphite output may be a solution.