Improve api_paste_config file searching

Currently, if the file defined by api_paste_config is not found under
working directory, it will use etc/ceilometer/api_paste.ini instead,
this is because at that time, devstack doesn't support the new config
file. After patch I57165d588da35f13b2d22629b48dc087502598a7 is merged,
this workaround is no longer needed, and we should use advanced
find_file() provided by oslo.config to search config file.

Meanwhile, if the file is found under working directory, we should use
absolute/relative path instead of bare base name, otherwise, the
deploy.loadapp will not work.

Change-Id: I9bcb7b69a0159d548401de1c36f773a1965722b6
Closes-Bug: #1329126
Merge "Improve api_paste_config file searching"