ec2 modules: Move more responsibility to common EC2 module

Moved `AWS_REGIONS` into `ec2` module
Created `ec2_connect` method in `ec2` module
Updated modules able to use `ec2_connect` and `AWS_REGIONS`


As mentioned on ansible-devel.
So far I've tested `ec2`, `ec2_tag` and `ec2_vol` - and all works as expected. 
I'll add further comments as other tests are made but if anyone wishes to assist, let me know.