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
```

this probably makes more sense as an instance variable... not super relevant in our current use case but I'm not sure I see the value in this being a class variable and would rather avoid a surprise down the road :)

something like 
```
def __init__(self, *args, **kwargs):
    self.partition = None
    super(BasePlugin, self).__init__(*args, **kwargs)
```