Vagrant VM memory environment variable.

Has Vagrant try to read `KUBERNETES_MEMORY` environment varilable when setting
$vm_mem.

fixes #3792

Vagrantfile

@@ -50,11 +50,11 @@ else # sorry Windows folks, I can't help you
   $vm_cpus = 2
 end
 
-# Give VM 1024MB of RAM
+# Give VM 1024MB of RAM by default
 # In Fedora VM, tmpfs device is mapped to /tmp.  tmpfs is given 50% of RAM allocation.
 # When doing Salt provisioning, we copy approximately 200MB of content in /tmp before anything else happens.
 # This causes problems if anything else was in /tmp or the other directories that are bound to tmpfs device (i.e /run, etc.)
-$vm_mem = 1024
+$vm_mem = (ENV['KUBERNETES_MEMORY'] || 1024).to_i
 
 Vagrant.configure(VAGRANTFILE_API_VERSION) do |config|
   def customize_vm(config)

docs/getting-started-guides/vagrant.md

@@ -307,5 +307,14 @@ You can control the number of minions that are instantiated via the environment
 export NUM_MINIONS=1
 ```
 
+#### I want my VMs to have more memory !
+
+You can control the memory allotted to virtual machines with the `KUBERNETES_MEMORY` environment variable.
+Just set it to the number of megabytes you would like the machines to have. For example:
+
+```
+export KUBERNETES_MEMORY=2048
+```
+
 #### I ran vagrant suspend and nothing works!
 ```vagrant suspend``` seems to mess up the network.  It's not supported at this time.