introduce sysrepo group for sysrepo-related processes

Sysrepo made some changes to file permissions and because of that
rousette began to fail because of insufficient permissions to open
sysrepo shm files.
It started to become tricky to manage all the stuff we decided to go
with what their README says [1], i.e., compile sysrepo with flags that
make sysrepo to create files with a specified group (sysrepo) and with
default umask (0007).
This is done via patches to our buildroot tree.

All sysrepo-related services then must run under sysrepo group so they
have access to sysrepo internal files.

Depends-on: https://gerrit.cesnet.cz/c/CzechLight/dependencies/+/5748
Change-Id: I257b9016bf7ca2af20f5627a2fe1b79b077c0232
diff --git a/package/velia/velia-health.service b/package/velia/velia-health.service
index f779bf6..05f66c2 100644
--- a/package/velia/velia-health.service
+++ b/package/velia/velia-health.service
@@ -9,6 +9,7 @@
 ExecStartPre=/bin/sh -c 'for COLOUR in red green blue; do echo none > /sys/class/leds/status:$${COLOUR}/trigger; echo 0 > /sys/class/leds/status:green/brightness; done'
 ExecStart=/usr/bin/veliad-health --appliance=czechlight-clearfog --systemd-ignore-unit=systemd-journal-upload.service
 ExecStopPost=/bin/sh -c 'for COLOUR in red green blue; do echo 0 > /sys/class/leds/status:$$COLOUR/brightness; done; [[ "$EXIT_CODE" == "exited" ]] && COLOUR="green" || COLOUR="red"; echo timer > /sys/class/leds/status:$$COLOUR/trigger; echo 256 > /sys/class/leds/status:$$COLOUR/brightness'
+Group=sysrepo
 PrivateTmp=yes
 PrivateDevices=yes
 ProtectSystem=yes