AWS: Eucalyptus 4.x compatibility fix

For Eucalyptus 4.0.1 requesting metadata seem to work differently as with EC2.

In Euca:
> curl http://169.254.169.254/2009-04-04
<?xml version="1.0"?><Response><Errors><Error><Code>404 Not Found</Code><Message>unknown</Message></Error></Errors><RequestID>unknown</RequestID></Response>

> curl http://169.254.169.254/2009-04-04/
dynamic
meta-data
user-data

In AWS EC2
> curl http://169.254.169.254/2009-04-04
"" (zero bytes)

> curl http://169.254.169.254/2009-04-04/
dynamic
meta-data
user-data

As the isAvailable() function in metadata.go tests only for error code it fails in Euca.