openstack/common: Disable package even when enable isnt the first keyword

Currently, services are disabled only if enable or disable is the first
keywork in the systemctl command. For example : 
```
  systemctl enable myservice.service
```
Unfortunately, enable or disable isn't always the first word. For example,
Puppet enable its service with this command :
```
  systemctl --no-reload enable puppet.service
```
Hence the current script does not work anymore. This patch makes the
`install_packages_disabled` works no matter where is the enable keyword
located.