When initializing for the in-memory database (NSS_NoDB_Init) the
internal slot is read-only so a lot of actions (PK11_ImportCert)
fails. Instead of that we create a new cert/key database inside
the tmp directory and delete it on exit. This way there are no
limitations and all the actions perform as expected.

Change-Id: Iadec5dd8f3459be56ba57d077057eacf3e0797fc
Reviewed-on: https://gerrit.libreoffice.org/65765
Tested-by: Jenkins
Reviewed-by: Tomaž Vajngerl <quikee@gmail.com>