Add registry authentication variables to Host Configuration page

[thomastaylor312]

The list of all the host configuration variables should contain the OCI_REGISTRY_USER and OCI_REGISTRY_PASSWORD variables so users can figure out how to set up their host to talk to a private registry

[stevelr]

We should make sure there's a way to associate the oci url with the credentials. With three separate variables for url, name, and password, if there is more than one url how do you know which password goes with which url? Other schemes that might work:

• one variable per set of url, optional name and password, separated by a delimiter. Eg: http://example.com/foo:1.1.1@alice@password123!

• change the var name to include the domain, eg oci_user_example_com=alice

[thomastaylor312]

We can probably follow a similar pattern to what is in the config service interface