ec2.py configuration overrides and location flexibility

This commit introduces the following features:

  - 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.
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.

Documentation and examples in the updated ec2.ini file. Building upon
the previous commit in this branch, each of these settings can be
overridden  with environment variables.
enhancements to ec2