Improve hostname and domain name validation
DNS plugin did not check DNS zone and DNS record validity and
user was thus able to create domains like "foo bar" or other
invalid DNS labels which would really confuse both user and
bind-dyndb-ldap plugin.
This patch at first consolidates hostname/domain name validators
so that they use common functions and we don't have regular
expressions and other checks defined in several places. These
new cleaned validators are then used for zone/record name
validation.
https://fedorahosted.org/freeipa/ticket/2384