When removing a XenServer host from a domain, the domain config is not properly cleaned up

Steps to reproduce:

1. Join XenServer 7 host to a domain
2. Remove XenServer 7 host from a domain
3. Examine /etc/hosts

You'll see that the domain's hostname is incorrectly left in the /etc/hosts file, this can cause problems with various software and configurations, this should be removed when leaving a domain.

For example, host not on any domain:

127.0.0.1 s1-b9 localhost localhost.localdomain localhost4 localhost4.localdomain4

Host joined to domain:

127.0.0.1 s1-b9.my.cool.domain.com s1-b9 localhost localhost.localdomain localhost4 localhost4.localdomain4

Host removed from domain:

127.0.0.1 s1-b9.my.cool.domain.com s1-b9 localhost localhost.localdomain localhost4 localhost4.localdomain4