`update_p4prom.sh` & `update_node.sh`: Add vmagent Memory Limit (20%) to Generated systemd Service

[jackclucas]

When update_p4prom.sh installs vmagent, it should set a safe memory limit by default.
vmagent’s default memory.allowedPercent=60 can reserve excessive memory (e.g., 4.8 GB on an 8 GB system), leaving insufficient memory for p4d and causing OOM kills.

Fix:

Add -memory.allowedPercent=20 to the vmagent ExecStart block.

[rcowham]

@jackclucas Looking forward to the PR to fix this!