Retrieve default SigLevel value from backend after initial setting

Retrieve default SigLevel value from backend after initial setting

This ensures we grab and use the library default once we have processed
the global SigLevel setting, but before processing the repo-specific
settings. This means the following two configs will now evaluate the
same, as the backend currently defaults to 'Optional':

Config 1:
    [options]
    # nothing here
    [repo]
    SigLevel = TrustAll

Config 2:
    [options]
    SigLevel = Optional
    [repo]
    SigLevel = TrustAll

Signed-off-by: Dan McGee <dan@archlinux.org>