set default for SERVICED_ADMIN_GROUP depending on Ubuntu or RHEL7

DEMO on Ubuntu:
```
~/src/europa/src/golang/src/github.com/control-center/serviced/pkg
# plu@plu-9: uname -a
Linux plu-9 3.13.0-24-generic #47-Ubuntu SMP Fri May 2 23:30:00 UTC 2014 x86_64 x86_64 x86_64 GNU/Linux
~/src/europa/src/golang/src/github.com/control-center/serviced/pkg
# plu@plu-9: SERVICED_ADMIN_GROUP=$(grep -q DISTRIB_ID=Ubuntu /etc/*-release && echo sudo || echo wheel)
~/src/europa/src/golang/src/github.com/control-center/serviced/pkg
# plu@plu-9: echo $SERVICED_ADMIN_GROUP
sudo
```
This will not work for RHEL-based platforms that consume this via systemd