OpenStack packstack installer hangs at 'Preparing servers...'

Issue

• The OpenStack packstack installer hangs at the Preparing servers... stage:

[root@controller /]# packstack --answer-file=/root/answers.txt 
Welcome to Installer setup utility
Packstack changed given value  to required value /root/.ssh/id_rsa.pub

Installing:
Clean Up...                                              [ DONE ]
Setting up ssh keys...                                   [ DONE ]
OS support check...                                      [ DONE ]
Adding pre install manifest entries...                   [ DONE ]
Installing time synchronization via NTP...               [ DONE ]
Adding MySQL manifest entries...                         [ DONE ]
Adding QPID manifest entries...                          [ DONE ]
Adding Keystone manifest entries...                      [ DONE ]
Adding Glance Keystone manifest entries...               [ DONE ]
Adding Glance manifest entries...                        [ DONE ]
Adding Cinder Keystone manifest entries...               [ DONE ]
Installing dependencies for Cinder...                    [ DONE ]
Checking if the Cinder server has a cinder-volumes vg... [ DONE ]
Adding Cinder manifest entries...                        [ DONE ]
Adding Nova API manifest entries...                      [ DONE ]
Adding Nova Keystone manifest entries...                 [ DONE ]
Adding Nova Cert manifest entries...                     [ DONE ]
Adding Nova Compute manifest entries...                  [ DONE ]
Adding Nova Network manifest entries...                  [ DONE ]
Adding Nova Scheduler manifest entries...                [ DONE ]
Adding Nova VNC Proxy manifest entries...                [ DONE ]
Adding Nova Common manifest entries...                   [ DONE ]
Adding OpenStack Client manifest entries...              [ DONE ]
Adding Horizon manifest entries...                       [ DONE ]
Preparing servers...                                     [ DONE ]