ipalib: Handle percent signs in saved values
    
    Turn off string interpolation on the FileStore class to avoid
    exceptions when a value to be saved contains a percent sign (%).
    The underlying SafeConfigParser that is used interprets percent
    signs as placeholders to be interpolated which then causes an
    exception as the placeholder isn't properly formatted.
    
    ipa-client-install uses the FileStore class to backup certain
    values that it overwrites as part of the installation. If those
    pre-existing, backed-up values contained a percent sign,
    ipa-client-install would throw an exception and thus prevent
    installation.
    
    https://pagure.io/freeipa/issue/9085
    
    Reviewed-By: Rob Crittenden <rcritten@redhat.com>