Glance URL should not be required for OpenStack providers

[n1zyy]

We presently mark Glance URL as a required field when adding an OpenStack provider.

However, the field is not actually required -- indeed, some providers don't support it. So really, we need to do two things:

1. Remove the validation that requires this field.
2. Ensure that our build code is capable of handling a situation in which this field is blank.