Correct os-collect-config.cfn.path to be string.

Specified as a list, it gets translated literally into
["NovaCompute0"] in /etc/os-collect-config.conf. This is not
desirable and results in 500 errors because ["NovaCompute0"] does
not exist.

Change-Id: I7dc01937d73c0c0e5cea70f5a3202d0e43c4caed