Correct update extension point's check_func for v3 server's controller

When loading update extension point, it should check the extension has
'server_update' function. But current code check with wrong function
name 'server_resize'. And this patch adds testcase for servers extension
point.

Change-Id: I2bee7ff306b3337d24047e471fac9485fec9a6f7
Closes-bug: #1240351
Merge "Correct update extension point's check_func for v3 server's controller"