Optimize Vagrant setup

- define grains to match optimized gvasalt repository
- define minion configuration in separate file
- drop custome salt/bootstrap.sh
- adapt Vagrantfile to use the custom grains file
- increase memory size of created VM to allow the salt provisioning to
  succeed

Vagrantfile

@@ -12,8 +12,11 @@ Vagrant.configure(2) do |config|
   config.vm.synced_folder "../gvasalt/states/", "/srv/salt/"
   config.vm.synced_folder "../gvasalt/pillar/", "/srv/pillar/"
 
+  config.vm.provider :libvirt do |libvirt|
+      libvirt.memory = 1024
+  end
+
   config.vm.provision :salt do |salt|
-      salt.bootstrap_script = "salt/bootstrap.sh"
       salt.bootstrap_options = "-x python3"
       salt.minion_id = "gvaldap"
       salt.masterless = true
@@ -21,5 +24,6 @@ Vagrant.configure(2) do |config|
       salt.verbose = true
       salt.colorize = true
       salt.log_level = "warning"
+      salt.grains_config = "salt/grains"
   end
 end