[master]: Narrow t2 selection criteria. (#44)

This narrows the match for things considered to be t2 instances to only
those types that start with "t2.". Previously there was a class of
(invalid) instance types, such as "2b.something" or "t5.something" that
would cause an error on boolean parsing by matching one regex and not
the other.

main.tf

 locals {
-  is_t2_instance_type = "${replace(replace(var.instance_type, "/^[^t2].*/", "0"), "/^t2.*$/", "1")}"
+  is_t2_instance_type = "${replace(var.instance_type, "/^t2\\..*$/", "1") == "1" ? "1" : "0"}"
 }
 
 ######