Support VPC tenancy restrictions and filters for DHCP options

This pull request adds support for 2 VPC related features that aren't currently exposed by Boto:

* Filter DHCP options in get_all_dhcp_options() as described in http://docs.aws.amazon.com/AWSEC2/latest/APIReference/ApiReference-query-DescribeDhcpOptions.html
* Allow restricting VPC tenancy to only allow dedicated instances, as described in <http://docs.aws.amazon.com/AWSEC2/latest/APIReference/ApiReference-query-CreateVpc.html>. Note that although the documentation says that "instanceTenancy" is lower camel case, I verified by usage that it should be upper camel case "InstanceTenancy".