ec2 inventory: Make it possible to group instances via their tags using group names explicitly named as <tag>=<value>

There are plenty of times where you'd like to do something like the following with ansible:

ansible -i ./ec2.py -a <some command> <tag>=<value>

-or-

ansible-playbook -i ./ec2_inventory/ playbooks/foo.yml  -l mytag=foo 
ansible-playbook -i ./ec2_inventory/ playbooks/baz.yml -l mytag=baz

Recent changes to the EC2 inventory plugin added this specifically as host variables, but this change does the same as explicit group names so that the above is possible.