boto_asg - add support for use of security group names in launch configs

### Overview:

This pull request will allow the use of a security group names within the ```boto_asg``` state module when creating Auto Scaling Groups within a VPC.

### Previous Behavior:
When given a state file that looks like the following:
```
Ensure gateway security group exists:
  boto_asg.present:
    - name: test-vpc
    - availability_zones:
      - us-east-1c
      - us-east-1d
    - min_size: 0
    - max_size: 0
    - desired_capacity: 0
    - region: us-east-1
    - vpc_zone_identifier:
      - subnet-61b47516
      - subnet-a042a1f9
    - tags:
      - key: 'Name'
        value: 'test'
        propagate_at_launch: true
    - launch_config_name: test
    - launch_config:
      - image_id: ami-a427efcc
      - security_groups:
        - sg-c4b3cea1
        - gateway
        - test-2
```
The boto_asg state would fail to execute due to a boto failure - the security group names aren't allows by Amazon's API because they lack specificity. As an example, the group ```gateway``` could be in multiple VPCs or in EC2-Classic and EC2-VPC - resulting in a condition where the launch configuration could not be created.

### New Behavior:
Behavior has been modified slightly to allow for use of security group names within a boto_asg.present State Definition. Description Below.
1. If one or more VPC Zone Identifiers are given in an Auto Scaling Group Configuration, the boto_asg state module will call ```boto_vpc.get_subnet_association``` (newly added module/function) to determine which VPC the Auto Scaling Group is associated with.
2. If associated with a VPC, the boto_asg module will determine if there are any security groups defined in the Auto Scaling Group's Launch Configuration. If there are Launch Configuration <-> Security Group associations, the boto_asg state module will convert all Security Group Names to Security Group IDs using ```boto_secgroup.get_group_id``` (newly added module/function).
3. The new Auto Scaling Group and Launch Configuration will be built.

Additional Changes:
1. Unit Testing for newly added ```boto_secgroup.get_group_id``` and ```boto_vpc.get_subnet_association``` modules and functions.