cleanup /tmp/serviced-root/var correctly before startup

ISSUE - we have no perms to read /tmp/serviced-root/var/isvcs/ so nothing is removed:
```
# plu@plu-9: ls -ld /tmp/serviced-root/
drwxr-x--- 2 root root 4096 Sep 10 08:56 /tmp/serviced-root/

# plu@plu-9: sudo rm -fr /tmp/serviced-root/var/isvcs/*

# plu@plu-9: sudo tree -L 2 /tmp/serviced-root/
/tmp/serviced-root/
└── var
    ├── isvcs
    └── volumes
3 directories, 0 files

# plu@plu-9: sudo rm -rf /tmp/serviced-root/var

# plu@plu-9: sudo tree -L 2 /tmp/serviced-root/
/tmp/serviced-root/
0 directories, 0 files
```

DEMO:
```
# plu@plu-9: make smoketest
cd isvcs && make
...
===== SUCCESS =====
Service shell ran successfully
===================
b77c6bdcafb2
d1a4ce88f175
ef655f76a310
a3b6fad0df98
ce55a2b8a0e2
91ba060dc301
26c71aac14d4
/tmp/serviced-root

0 directories, 0 files

```