add function to find vpc based on tags or cidr_block, returns vpc_id

Function locates VPC based on the specified criteria (cidr block or tags (implicitly name of the vpc)) 

Sample usage:

```
salt-call -l debug --skip-grains --local 'boto_vpc.find' tags='{"Name": "Dev Test VPN for Michal"}'
```

or 

```
salt-call -l debug --skip-grains --local 'boto_vpc.find' cidr='10.0.0.0/16'
```