add --hypervisor option

the option is required to launch an instance with ISO image attached,
even if only single hypervisor is available.
Merge branch 'hypervisor-in-server-create' of https://github.com/reallyenglish/knife-cloudstack-fog into reallyenglish-hypervisor-in-server-create

Conflicts:
	lib/chef/knife/cloudstack_server_create.rb
Merge branch 'reallyenglish-hypervisor-in-server-create'