certificate_authority: Rename "Client" as "Service".

Certificates can be used by both clients and servers over an SSL link, so
Client is misleading.

slapos/certificate_authority/doc/P-SLAPOS.Certificate.Authority.Plantuml.Sequence.Diagram.Signing.Request.txt

@@ -4,7 +4,7 @@ title Automated Certificate Authority Service
 
 actor User
 actor Trusted
-actor Client
+actor Service
 autonumber
 
 == Signing Request Submission ==
@@ -76,6 +76,6 @@ end
 
 == Certificate Validity Check ==
 
-Client -> Authority : GET (optional: with OCSP parameter)
-Authority --> Client : Certificate revocation list
+Service -> Authority : GET (optional: with OCSP parameter)
+Authority --> Service : Certificate revocation list
 @enduml