compute: create default firewall rules

[stephenplusplus]

Related http://stackoverflow.com/questions/31509722/how-do-i-enable-http-traffic-for-gce-instance-templates
Fixes stephenplusplus/gcloud-deploy#4

We had a "maybe-bug" in our code that could lead to a user thinking they made a VM that accepts HTTP and/or HTTPS connections, but in actuality doesn't.

My theory about how the Dev Console UI works:

If you create an Instance and select "Allow HTTP traffic" and/or "Allow HTTPS traffic", it will create firewall rules in the default network: default-allow-http and default-allow-https. The VM is created with tags to match http-server and https-server, which are the target tags on the firewall rules.

When I developed the behavior of zone.createVM() that accepts config.http = true and config.https = true, I assumed all project's networks had these firewall rules by default. But, if my theory is right, I only had them because I used the UI to create an http/https VM before, so my project had the rules.

So, this PR will attempt to create default-allow-http and default-allow-https (when that functionality is asked for), and if they already exist, it just moves on and creates the VM. If they don't exist, it creates them, then the VM. In other words, this should basically work like the Dev Console UI does (if my theory is correct).

[jgeewax]

LGTM

[stephenplusplus]

@callmehiphop PTAL

[callmehiphop]

Looks good :)