Replace utils.config_find with CONF.find_file

As part of transfer for using common code function 'config_find'
from 'manila.utils' module replaced with 'find_file' method of 'oslo.cfg.ConfigOpts' class.

For unittest compatibility was added new conf fixture with specifying abs path
to used ini file.

Partially-implements blueprint use-common-code

Change-Id: Iad85d1cafc1e47d85fa9d89edfe56e580a848c14
Merge "Replace utils.config_find with CONF.find_file"