Add support for a 'no manage' tag (aws#726)

* Add support for a 'no manage' tag

This tag results in an ENI being unmanaged by the ipamd plugin. This
allows someone to create an ENI, associate it with an instance, and then
use eks per normal while also using that ENI for whatever.

* Update tests for new changes

* Remove 'unmanaged eni' metric

* Update readme with docs about CLUSTER_NAME and no_manage

* Correctly indent tag sub-headers

* Remove trailing '.'

* Account for unmanaged ENIs in allocation check

README.md

@@ -474,4 +474,4 @@ instructions [here](https://aws.amazon.com/security/vulnerability-reporting/) or
 
 ## Contributing
 
-[See CONTRIBUTING.md](./CONTRIBUTING.md)
+[See CONTRIBUTING.md](./CONTRIBUTING.md)