KCM: Pass confdb context to the ccache db initialization
    
    Resolves:
    https://pagure.io/SSSD/sssd/issue/3386
    
    The libsecrets back end needs to read the quota options from confdb,
    therefore it needs to know the section and access the confdb handle.
    
    These parameters are unused for other ccache back end types, but they
    are harmless and IMO it makes more sense to keep the ccache back end
    abstract.
    
    Reviewed-by: Michal Židek <mzidek@redhat.com>