Add exact_count and count_tag to the ec2 module.

Adds the exact_count and count_tag parameters to the ec2 module. 

Tags can be a dictionary, a list or a string. If only tag names are given, only their presence is evaluated. If values are given, values will also be matched.

Examples:
```
        count_tag: '{"db":"postgres"}'
```
```
        count_tag:
            Name: idempotent-instance
            foo: barbazbang
            empty:
```
```
        count_tag: foo
```
```
        count_tag:
            foo:
```