Correct configuration of o-c-c collectors

o-c-c registers it's "collectors" config param as a MultiStrOpt, rather
than a ListOpt. The required configuration file format for this type of
option is:

     [DEFAULT]
     collectors = heat_local
     collectors = ec2
     collectors = ...

Change-Id: I35063c75781f855883b02e1e0438b8cc08475ced
Merge "Correct configuration of o-c-c collectors"