Increase disk issues timeout for alerts

Change-Id: I646a852be587598ff0866e5941d954a6ac1fdd08

linux/meta/prometheus.yml

     SystemDiskSpaceTooLow:
       if: 'predict_linear(disk_free[1h], 8*3600) < 0'
       {% raw %}
-      for: 5m
+      for: 15m
       labels:
         severity: warning
         service: system
     SystemDiskInodesTooLow:
       if: 'predict_linear(disk_inodes_free[1h], 8*3600) < 0'
       {% raw %}
-      for: 5m
+      for: 15m
       labels:
         severity: warning
         service: system