Simplify validation on #source

Before:
```
irb(main):001:0> MiqProvisionRequest.create!
ActiveRecord::RecordInvalid: Validation failed: Source must have valid template, Vm template must have valid VM (must be in vmdb), Userid must have valid user
```

After:
```
irb(main):001:0> MiqProvisionRequest.create!
ActiveRecord::RecordInvalid: Validation failed: Source can't be blank, Userid must have valid user
```

app/models/miq_provision_request.rb

@@ -13,8 +13,7 @@ class MiqProvisionRequest < MiqRequest
   validates_inclusion_of :request_state,
                          :in      => %w(pending provisioned finished) + ACTIVE_STATES,
                          :message => "should be pending, #{ACTIVE_STATES.join(", ")}, provisioned, or finished"
-  validates_presence_of  :source_id,      :message => "must have valid template"
-  validate               :must_have_valid_vm
+  validates :source, :presence => true
   validate               :must_have_user
 
   default_value_for :options,      :number_of_vms => 1
@@ -39,10 +38,6 @@ def self.new_request_task(attribs)
     klass.new(attribs)
   end
 
-  def must_have_valid_vm
-    errors.add(:vm_template, "must have valid VM (must be in vmdb)") if vm_template.nil?
-  end
-
   def set_description(force = false)
     prov_description = nil
     if description.nil? || force == true