Support VPC tenancy restrictions and filters for DHCP options

[mbarrien]

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".

[danielgtaylor]

This looks great, however can you please add a unit test for the new parameters here? https://github.com/boto/boto/blob/develop/tests/unit/vpc/test_vpc.py

[mbarrien]

May take a while for me... this is my first time in Boto code and Boto's test framework. Keep in mind no existing unit test code exists at all for either of the 2 affected methods (or for most of the vpc module) in its current state before these modifications.

[mbarrien]

Added unit tests for new parameters (aka created unit tests for the first time for the functions). While I was in there, converted existing tests that used duck-typing to use the seemingly more official AWSMockServiceTestCase.

Please re-review and consider for merging.

[danielgtaylor]

This looks great! Thanks for writing the tests 👍