Mapping, filtering and overriding in ec2.py inventory script

Tag name mapping and host filtering capabilities added
    
This feature commit provides new configuration settings to do the following:
    
* Map a tag name/value to a different group name to the default tag_<key>_<value>
* Specify that the value for a given tag is a comma delimited list to be unwrapped such that it simulates having multiple tags with the same key but different values (example: Role:webserver,cacheserver)
* Specify that hosts in the resultant inventory should be limited to those that have a given tag set to a given value. Useful for limiting inventory only to those hosts in a given VPC, for example.

In addition it is now possible to override ec2.ini settings with environment variables, and the search path for ec2.ini has been enhanced:

* The ini file variables can be overridden by setting environment labels
* The location of the ini file can be specified in environment labels and will also be sought in the current working dir prior to falling back to the original behaviour which is to look in the directory in which the ec2.py script is located.

Documentation and examples in the updated ec2.ini file.