Changes

add PEM cert as used in docs/osmo-smdpp.rst

Add PEM version of smdpp-data/certs/DPtls/CERT_S_SM_DP_TLS_NIST.der

A CERT_S_SM_DP_TLS_NIST.pem file is referenced in docs/osmo-smdpp.rst --
nginx apparently cannot use DER certs, so it is convenient for beginners
if the example from the docs just works without having to know that:

The added file was produced using

    openssl x509 -inform DER -in CERT_S_SM_DP_TLS_NIST.der -outform PEM -out CERT_S_SM_DP_TLS_NIST.pem

Change-Id: I41ba6ebacb71df0eb8a248c0c3c9ccd709718d74