Variables should be validated before instances created

[vvaldez]

If some variables such as dns_domain are not set, certain tasks will fail that depend on this such as:

TASK [hostnames : Setting FQDN Fact] *******************************************
fatal: [casl-vvaldez-1468605453-master1]: FAILED! => {"failed": true, "msg": "'dns_domain' is undefined"}
fatal: [casl-vvaldez-1468605453-nfs1]: FAILED! => {"failed": true, "msg": "'dns_domain' is undefined"}
fatal: [casl-vvaldez-1468605453-node1]: FAILED! => {"failed": true, "msg": "'dns_domain' is undefined"}
fatal: [casl-vvaldez-1468605453-node2]: FAILED! => {"failed": true, "msg": "'dns_domain' is undefined"}

This should be verified in a pre_task section.