engine: Import single certificate

GetProviderCertificate is run instead of GetCertificateChain, and retrieves
the top certicate from the chain.

The user approves the certificate, and then it is imported using the new
ImportProviderCertificate command which gets an encoded payload of the certificate
as parmater.

Change-Id: Ic9adb21ded6e6d9fb09fc68331872c1cd88f88a9
Signed-off-by: Yair Zaslavsky <yzaslavs@redhat.com>