Ec2tags selector

2 commits.
1. Adds max_checks and check_interval as top level options. This cuts down on synapse cpu usage. 
    We run in under supervisord, so max_check = 10 would exit after those many loops.

2. Adds a fancier selector  with a @discovery['selector'] option.
    This option as actually eval'd with 'i' being the AWSInstance variable.

We are using this in production.