use vpc-id and zone instead of subnet-id for ec2

Signed-off-by: Evan Hazlett <ejhazlett@gmail.com>
add default zone for amazonec2

Signed-off-by: Evan Hazlett <ejhazlett@gmail.com>
allow vpc-id, zone and subnet-id options for ec2

Signed-off-by: Evan Hazlett <ejhazlett@gmail.com>
Merge pull request #98 from ehazlett/ec2-use-vpc

add vpc-id and zone options for ec2