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

Added documentation on profiles to EC2 dynamic inventory doc

Only tries to use profiles if --profile argument is given
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)