Allow ec2 inventory to use a boto profile

This allows the EC2 inventory plugin to be used with
the same configuration against different EC2 accounts

Profile can be passed using --profile variable or using
EC2_PROFILE environment variable e.g.

```
EC2_PROFILE=prod ansible-playbook -i ec2.py playbook.yml
```

Added documentation on profiles to EC2 dynamic inventory doc

Only tries to use profiles if --profile argument is given
or EC2_PROFILE is set to maintain compatibility will boto < 2.24.

Works around a minor bug in boto where if you try and use
a security token with a profile it fails (boto/boto#2100)

Allow inventory scripts to take additional arguments - e.g.
```
ansible-playbook -i 'ec2.py --profile prod' playbook.yml
```