Fix unit testing w/ go 1.7.1 and support enhanced wildcard dimensions

[andyday]

Required for all PRs:

• CHANGELOG.md updated (we recommend not updating this until the PR has been approved by a maintainer)
• Sign CLA (if not already signed)
• README.md updated (if adding a new plugin)

Changes

1. Fixed unit testing on go 1.7.1, see make test fails because of go vet #1696
2. Currently dimension wildcards use an all-or-nothing approach. Added support for partial wildcarding (ie. value = 'foo*' matches all values starting with foo)

[sparrc]

from what I can tell there appears to be overlap between #1885 and #1903. I would appreciate if one of you could open an issue with a full writeup on what exactly is wrong the cloudwatch plugin.

As this plugin is particularly difficult to reproduce problems on, please be sure to include examples of requests and responses, and links to documentation where necessary.

Please also provide concrete examples that illustrate what is wrong with the current behavior of the plugin.

If I am wrong and there isn't overlap, then feel free to open two separate issues. In either case I think they need to be seen and discussed before I can accept a PR changing the behavior of this plugin.