CertDB: add API for non-destructive initialization from PKCS#12 bundle
`create_from_pkcs12` method of CertDB was re-creating NSS database files
during PKCS#12 bundle import. This may cause bugs because the file permissions
could be re-set to wrong values causing subtle bugs. Modify the class API so
that the server cert chain can be imported while preserving existing FS
attributes.
https://fedorahosted.org/freeipa/ticket/6429
Reviewed-By: Jan Cholasta <jcholast@redhat.com>