Adding support for ansible_ec2_vpc_id correspondind to ansible_ec2_mac in ec2_facts.

This is a convenient correction for obtaining vpc-id as an attribute of ec2_fact. 
For instance, to get vpc-id in the conventional method:
{{ ansible_ec2_network_macs_06_11_22_33_44_55_vpc_id }}
in this method:
{{ ansible_ec2_vpc_id }}.

Since mac refers to the eth0 device, in the case of not multiple network interfaces, it is useful.