Fixed ability to launch spot instance within a VPC with custom VPC security group

Fixes:
Status Code: 400, AWS Service: AmazonEC2, AWS Request ID: 6079cf60-3c23-4ea7-935a-807bd5f81b95, AWS Error Code: InvalidParameterCombination, AWS Error Message: The parameter groupName cannot be used with the parameter subnet
Merge pull request #63 from mriddle/master

[JENKINS-19301] Fix: spot instance feature cannot be used within VPC