repos validator needs to handle missing urls

[nephomaniac]

It seems that if any of the following Env attributes are not defined, than the value 'None' is appended to a list of URLs to test.
['default-img-url', 'euca2ools-repo', 'eucalyptus-repo', 'init-script-url', 'post-script-url']

This causes an exception during Request(url) which causes the validation to halt.
Suggesting the following:

• Mandatory URL such as eucalyptus-repo and euca2ools-repo should fail if not defined. We can ignore all other undef'd urls above.
• If an URL is defined but fails the request test, than a failure should be displayed regardless of the url.
• Also not sure if we should raise exceptions for these or just display these as failures? An exception currently halts any other urls from being validated.