vmdriver: adds 'hypervisor_type' configuration parameter

pillar/vmdriver.sls

 vmdriver:
   repo_name: https://git.ik.bme.hu/circle/vmdriver.git
   repo_revision: master
+  hypervisor_type: kvm

salt/vmdriver/files/postactivate

 export AMQP_URI=amqp://{{ pillar['amqp']['user'] }}:{{ pillar['amqp']['password'] }}@{{ pillar['amqp']['host'] }}:{{ pillar['amqp']['port'] }}/{{ pillar['amqp']['vhost'] }}
 export CACHE_URI={{ pillar['cache'] }}
 export LIBVIRT_URI=qemu:///system
-export HYPERVISOR_TYPE=kvm
+export HYPERVISOR_TYPE="{{ pillar['vmdriver']['hypervisor_type'] }}"
 export NATIVE_OVS=True