closes #1861

Let's align to InfluxDB 1.0 logging policy and log to systemd journal by
default.

closes #1732

Issue #1854

closes #1848
closes #1835

in this commit:

- centralize logging output handler.
- set global Info/Debug/Error log levels based on config file or flags.
- remove per-plugin debug arg handling.
- add a I!, D!, or E! to every log message.
- add configuration option to specify where to send logs.

closes #1786

this will prevent potential very large allocations due to a very large
chunk size send from a client.

fixes #1823

Due to quite real problem of generating vast number of data series through
mesos tasks metrics this feature is disabled until better solution is found.

Tagging values by executor_id can create quite a lot data series
in InfluxDB so we should stick to framework_id and server.

closes #1137

closes #1517

max-repetitions = 10 is the default of net-snmp utils according to
http://net-snmp.sourceforge.net/docs/man/snmpbulkwalk.html

retries = 3 is the default of gosnmp:
https://godoc.org/github.com/soniah/gosnmp#pkg-variables

Could deal with some parts of the performance issues reported
by #1665

Also consolidated the translation code to obtain all info with just 1 command execution.

Also split test command mocks out to their own file for cleanliness.

closes #1752

closes #1751

The default is 0 so we hit a division by 0 error and crash. This checks
ensure we will not crash and `log` and continue to let telegraf run

Also we set default allow pending message number to 10000

* Allow numeric and non-string values for tag_keys.

According to the go documentation the JSON deserializer only produces these
base types in output:
- string
- bool
- float64
- nil
With this patch bool, float64 and nil values get converted to a string when
their field key is specified in tag_keys. Previously the field was simply
discarded.

* Updated handling of nil for passing tests.

The automated tests are less than trivial to reproduece locally for me,
so I hope CircleCI wonn't mind...

* Updated changelog entries with PR and issue links.

* Code correctly handles colons in tag values now

* Modified existing datadog tag test to include a tag value containing a colon

closes #1789

* Add container state metrics to docker plugin.

* Update changelog.

closes #1773

closes #1775

speed up statsd parser

add statsd parsing benchmarks