certificate_authority: Start documenting the use-cases.

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

+@startuml
+title "Submitting a certificate signature request"
+
+
+actor User
+actor Trusted
+autonumber
+
+User -> Authority : POST /request
+alt Sining request passes all checks
+  Authority --> User : Signing request identifier
+else
+  Authority --> User : Error
+end
+Trusted -> Authority : GET /requests XXX: check plural with Romain
+Authority --> Trusted : List of pending signing requests with their identifiers
+Trusted -> Authority : GET /request with request identifier
+Authority --> Trusted : Signing request content
+alt Trusted agrees to prvoduce a signed certificate from the signing request
+  Trusted -> Authority : POST /sign with the signing request identifier
+  alt Sining request was still pending
+    Authority --> Trusted : Success
+  else
+    Authority --> Trusted : Not found
+  end
+else
+  Trusted -> Authority : DELETE /request with the signing request identifier
+  Authority --> Trusted : Ok
+end
+loop Until certificate obtained or request rejected
+  User -> Authority : GET /certificate with signing request identifier
+  alt Signing request was signed
+    Authority --> User : Certificate content
+  else
+    Authority --> User : Not found
+    opt User wants to check request is still pending
+      User -> Authority : GET /request with signing request identifier
+      alt Sining request is still pending
+        Authority --> User : Signing request content
+      else
+        Authority --> User : Not found
+      end
+    end
+  end
+end
+@enduml