Add EC2 tags to other system tags, to prevent them from overwriting other AWS tags.
Make collection of ec2 tags configurable.  Return empty tag list in case of error.
Code review feedback:
-Move config value handling to config.py
-Remove EC2 SourceTypeId
-Log EC2 tag retrieval exceptions
Merge pull request #789 from DataDog/agent_aws_tags_to_system

Agent aws tags to system