ipa: save cert as blob in the cache
    
    The IPA extdom plugin returns the user certificate base64 encoded.
    Before the IPA client can store it in the cache it must be decoded so
    that it is stored as a binary as the certificate from other sources.
    
    Reviewed-by: Jakub Hrozek <jhrozek@redhat.com>
    (cherry picked from commit cf89f552f06b95bd69d8c61aaa55a330a5d9f6e6 with
    backport fixes)