Ping fixes

Based on further testing it seems like AWS expects ICMP rules to be defined as type..-1. Strangely the new AWS CLI tools do not allow passing in a negative value, while the AWS web console always generates rules that are interpreted by fog as type..-1. Looking at other icmp types like 3 - destination unreachable I think the from and to ports may actually be interpreted by AWS as type and code respectively. It still seems strange to me that AWS would specify the code for echo request/reply as -1, since according to http://en.wikipedia.org/wiki/Internet_Control_Message_Protocol the code should be 0 in both cases. However, using -1 seems best since that is what works and what the web-ui generates. Upon further reflection -1 is probably just a null placeholder value.

Notably this PR replaces `:ping` with `:echo_request` + `:echo_reply`.