Description of problem:
When trigger an quick install against non-ha env, the output message hint "the cluster host name (Ansible variable openshift_master_cluster_public_hostname) is set by default to the host name of the single master", but the default choice is set to None in cli and installer.cfg.yml with two variables.

Version-Release number of selected component (if applicable):
atomic-openshift-utils-3.4.23-1.git.0.317b2cd.el7.noarch

How reproducible:
always

Steps to Reproduce:
1.trigger an quick install
# atomic-openshift-installer install

2.input host info(only one master) and select "n" for additional hosts.
Do you want to add additional hosts? [y/N]: n

It will output following message for user to make a choice:

You have chosen to install a single master cluster (non-HA).

In a single master cluster, the cluster host name (Ansible variable openshift_master_cluster_public_hostname) is set by default to the host name of the single master. In a multiple master (HA) cluster, the FQDN of a host must be provided that will be configured as a proxy. This could be either an existing load balancer configured to balance all masters on
port 8443 or a new host that would have HAProxy installed on it.

(Optional)
If you want to override the cluster host name now to something other than the default (the host name of the single master), or if you think you might add masters later to become an HA cluster and want to future proof your cluster host name choice, please provide a FQDN. Otherwise, press ENTER to continue and accept the default.

Enter hostname or IP address [None]: 

2. press enter


Actual results:
1)There will be two variables set but not one variable as described in hint "the cluster host name (Ansible variable openshift_master_cluster_public_hostname)".
2)The default value of cluster host name is set to "None" but not the host name of the single master which is not consistent with the hint in output message.

# cat installer.cfg.yml
<--snip-->
openshift_master_cluster_hostname: None
openshift_master_cluster_public_hostname: None
<--snip-->
 
Expected results:
The cluster host name and its default value should be more clear in hint message and choice of cli(Enter hostname or IP address [None]).