add ability to use profile for ec2 provider

[drts01]

• Enable use of profiles
• Enable use of STS (required for accounts with multifactor-authentication)
• Use boto3 instead of boto in bootstrapvz/providers/ec2/tasks/connection.py

Addresses Issue #420

[andsens]

This cannot be merged as is right now, because it removes support for access/secret token. It would break a lot of peoples manifests. You will also need to document the ability to use the profile parameter in the EC2 docs.
Linting also fails right now:

bootstrapvz/providers/ec2/tasks/connection.py:52:13: E123 closing bracket does not match indentation of opening bracket's line
bootstrapvz/providers/ec2/tasks/connection.py:59:21: E122 continuation line missing indentation or outdented

:-(

[drts01]

I have not removed the ability to read in access/secret token, https://github.com/andsens/bootstrap-vz/pull/423/files#diff-df81a97bd0bc86ad6a46a564ff3dd58dR77. It takes precedence over any other creds.

[drts01]

corrected the styling and updated docs.

[andsens]

You're right. I misread the code. Looks good! Merging...