Make nova CLI use term "server" where possible

nova client was using terms "server" and "instance" interchangeably
which was very confusing.

This patch uses "server" where possible without touching the API.

Note that there are actions (instance-action, instance-action-list) and
arguments (--instance-name, --num-instances, ...) with "instance" in their
name which can't be changed easily.

Closes bug 1156648

Change-Id: I0570aa1c4dc9ad1852af307a2a4896f16073a6ab