Make docstring of servers consistent

Docstring of service clients is very inconsistent, but we have a
plan to use it for generating API doc.
This patch makes docstring of servers consistent for fitting to
http://developer.openstack.org/api-ref-compute-v2.1.html
as a first step.

Change-Id: Icb51c2cf189fc17a91dcac5b95dc4a27afeac543
1 file changed