TT#50752 Add ngcp-service aliases in systemd service files

These are names used in the ngcp-service nsservices.yml file, adding
them here makes using the system more consistent.

Change-Id: I66b0149cbfe70d2260a6c50617a52e53604256da

debian/ngcp-rtpengine-daemon.service

@@ -14,3 +14,4 @@ ExecStopPost=/usr/sbin/ngcp-rtpengine-iptables-setup stop
 
 [Install]
 WantedBy=multi-user.target
+Alias=rtpengine.service

debian/ngcp-rtpengine-recording-daemon.service

@@ -13,3 +13,4 @@ ExecStart=/usr/sbin/rtpengine-recording -f -E --pidfile /run/ngcp-rtpengine-reco
 
 [Install]
 WantedBy=multi-user.target
+Alias=rtpengine-recording.service