Fix no implicit conversion to rational from nil

Before this change if the boottime was nil, the check_ready
loop would exit on Time.now - host['boottime'] with a TypeError
in jruby. The boottime is nil when the power is Off so moving that check
earlier should catch that bug.

spec/unit/pool_manager_spec.rb

@@ -322,6 +322,7 @@ EOT
 
       context 'is turned off' do
         before(:each) do
+          host['boottime'] = nil
           host['powerstate'] = 'PoweredOff'
         end