packaging: missing python3 libvirt dependency for CentOS7 (#4124)

Missing python3 libvirt bindings on CentOS7 effectively break security groups.
There are 0 firewall rules added. The agent logs report:

```2020-06-02 10:58:34,346 DEBUG [kvm.resource.LibvirtComputingResource] (main:null) (logid:) Traceback (most recent call last):  File "/usr/share/cloudstack-common/scripts/vm/network/security_group.py", line 26, in <module>    import libvirtModuleNotFoundError: No module named 'libvirt'
```

Signed-off-by: Rohit Yadav <rohit.yadav@shapeblue.com>

Author: NuxRo

packaging/centos7/cloud.spec

@@ -110,6 +110,7 @@ Requires: iproute
 Requires: ipset
 Requires: perl
 Requires: libvirt-python
+Requires: python36-libvirt
 Requires: qemu-img
 Requires: qemu-kvm
 Provides: cloud-agent