sumaform should use terraform 0.8.x newer features

[moio]

REQUIRES: https://github.com/moio/sumaform/issues/71

In many places throughout the codebase we use 0.7 language hacks to simulate conditionals, which were added in 0.8:

https://github.com/moio/sumaform/blob/master/modules/libvirt/client/main.tf#L17-L18
https://github.com/moio/sumaform/blob/master/modules/libvirt/host/main.tf#L8
https://github.com/moio/sumaform/blob/master/modules/libvirt/host/main.tf#L15
https://github.com/moio/sumaform/blob/master/modules/libvirt/host/main.tf#L49
https://github.com/moio/sumaform/blob/master/modules/libvirt/host/main.tf#L72
https://github.com/moio/sumaform/blob/master/modules/libvirt/minion/main.tf#L17-L18
https://github.com/moio/sumaform/blob/master/modules/libvirt/suse_manager/main.tf#L16
https://github.com/moio/sumaform/blob/master/modules/libvirt/suse_manager/main.tf#L32-L33
https://github.com/moio/sumaform/blob/master/modules/libvirt/suse_manager_proxy/main.tf#L27
https://github.com/moio/sumaform/blob/master/modules/openstack/host/main.tf#L35

All of these should be re-written for clarity.

We also make use of a different language hack to work around an issue with interpolated dictionaries, this should also be removed:

https://github.com/moio/sumaform/blob/master/modules/libvirt/base/main.tf#L44
https://github.com/moio/sumaform/blob/master/modules/libvirt/host/main.tf#L68

Note that the above should be tested thoroughly as the new behaviour was not really reliable in 0.8 betas.

Yet other hacks are used to work around the fact it was not possible to declare dependencies on whole modules (now it is). Those should be removed.

https://github.com/moio/sumaform/blob/master/modules/openstack/host/main.tf#L64

[moio]

Thanks @ncounter @mbologna for taking care of this!