Change 'admin_password' into v2 style for servers extension

This patch changes 'admin_password' to 'adminPass' that same
with v2 api. And also change the related unittest.

The differences between v2 and v3 are described on the wiki page
https://wiki.openstack.org/wiki/NovaAPIv2tov3.

Partially implements blueprint v2-on-v3-api

Change-Id: Ie5675f98bb15c55a191a207171e2ffe3be8e6deb
Merge "Change 'admin_password' into v2 style for servers extension"