Cache results of checking for ec2 default vpc
Merge pull request #962 from voidspace/cache-ec2-default-vpc

Cache results of checking for ec2 default vpc

The default-vpc of an ec2 account can't change during the lifetime of a juju environment (it requires days to change this and instances in 1 vpc can't communicate with instances in another via private network), so we can cache the result and avoid round tripping to Amazon every time we need it's Id or to check for its existence.