Many of the examples provided within documentation are using a single
dash for the command line arguments, but the telegraf executable
explicitly has two dashes.

There are also some inconsistencies with the ordering of the command
line argument examples.  I've ordered them so that the examples will
show: config, config-directory, input-filter, test

Mention that some metrics are available only at larger intervals than 5
minutes.  Update dead links to new locations and example config.

closes #1907

* Fix bug: too many cloudwatch metrics

Cloudwatch metrics were being added incorrectly. The most obvious
symptom of this was that too many metrics were being added. A simple
check against the name of the metric proved to be a sufficient fix. In
order to test the fix, a metric selection function was factored out.

* Go fmt cloudwatch

* Cloudwatch isSelected checks metric name

* Move cloudwatch line in changelog to 1.2 features

Also don't use named returns in fetchNamespaceMetrics since it's
non-standard for the rest of the codebase.

Reference #1670

closes #1197

* Use shared AWS credential configuration.

*  Cloudwatch dimension wilcards 

* Allow configuring cache_ttl for cloudwatch metrics.

* Allow for wildcard in dimension values to select all available metrics.

* Use internal.Duration for CacheTTL and go fmt

* Refactor to not use embedded structs for config.

* Update AWS plugin READMEs with credentials details, update Changelog.

* Fix changelog after rebasing to master and 0.13.1 release.

* Fix changelog after rebase.

closes #1085
closes #1086

closes #1078

Rebased commit of previously reviewed branch.
Added cloudwatch client Mock and more rich unit tests.

closes #935
closes #936