Allow control of hostname formatting

Allow control of hostname formatting

Currently, we use the heat default server names, which results in some
fairly unreadable hostnames due to the level of nesting in the templates.

e.g ov-sszdbj5rdne-0-bhseh65edxv6-Controller-zoqc6tlypbdp

Instead, we allow the user to specify a format string per role, defaulted
to a string which formats the name e.g <stackname>-controller-<index>

e.g overcloud-controller-0

Optionally additional hostname components (not replaced by heat) could be
added, such that deployment time customization of hostnames via firstboot
scripts (e.g cloud-init) may be possible.

Should anyone wish to maintain the old heat-generated names, they can pass
an empty string via these parameters, which heat will treat as if no "name"
property was provided to OS::Nova::Server.

Change-Id: I1730caa0c2256f970da22ab21fa3aa1549b3f90b