Add --partition parameter to allow mounting of partitioned volumes

This pull adds a --partition parameter to the linux block device plugin to facilitate mounting of volumes with partition tables. It also adds in proper handling of partitioned volumes to the linux volume plugin. It's currently being used in deployment to create HVM AMI's with the ansible and yum provisioners.

This follows the general concept outlined by @mtripoli in issue https://github.com/Netflix/aminator/issues/59

Usage example:
```
aminate -e ec2_ansible_amzn -n my_ami_name --partition 1 --vm-type hvm -B my-ami-id my/playbook.yml --debug
```

Makes sense, I'll make the change.