libvirt: Add Terraform variables for memory/CPU, bump master to 4GiB

My main dev environment is a Lenovo P50 with 64GB of RAM - I got
it specifically to run some large VMs (and/or many VMs) specifically
with OpenShift in mind.

First, default masters to 4096.

And for me, increasing RAM on my master to 8GB is a *very* noticeable speed improvement
and I think reliabilty; before I saw the apiserver be `OOMKilled`
sometimes, and `kswapd0` was constantly doing writeback.

These variables aren't bubbled all the way up to the documented
installer config, but one can now do e.g.:

```
$ env TF_VAR_tectonic_libvirt_memory=8192 TF_VAR_tectonic_libvirt_vcpu=4 ./bin/openshift-install create cluster --dir=osiris
```

Previously:

 - openshift#408
 - openshift#163

data/data/libvirt/main.tf

@@ -64,8 +64,8 @@ resource "libvirt_domain" "master" {
 
   name = "${var.tectonic_cluster_name}-master-${count.index}"
 
-  memory = "3072"
-  vcpu   = "2"
+  memory = "${var.tectonic_libvirt_memory}"
+  vcpu   = "${var.tectonic_libvirt_vcpu}"
 
   coreos_ignition = "${libvirt_ignition.master.id}"
 

data/data/libvirt/variables-libvirt.tf

@@ -32,3 +32,15 @@ variable "tectonic_libvirt_master_ips" {
   type        = "list"
   description = "the list of desired master ips. Must match tectonic_master_count"
 }
+
+variable "tectonic_libvirt_memory" {
+  type        = "string"
+  description = "RAM in MiB allocated to master and nodes"
+  default     = "4096"
+}
+
+variable "tectonic_libvirt_vcpu" {
+  type        = "string"
+  description = "CPUs allocated to master and nodes"
+  default     = "2"
+}