Change hostname variables

Use inventory_hostname in various places where hostname or
ansible_hostname were used.  inventory_hostname is automatically
assigned by ansible.  hostname was a variable I added before I
realized the availability of inventory_hostname.  ansible_hostname
is not appropriate because on the first pass through configuring a
new instance, it will not have been set yet to the desired hostname
for certain tasks (being the server's original hostname before we
set it).
Merge pull request #9 from dpla/improve-hostname

Change hostname variables