cfg: allow empty config values
cfg: allow empty config values

Fixes bug #1025522

Commit 83044a7 caused this to stop working in Quantum:

  api_extensions_path =

and could only be worked around with:

  api_extensions_path = ""

Change-Id: I8c1a57225a2c135e6baf567b8e71d61e974da4e2

openstack-dev/pbr,JioCloud/pbr,varunarya10/pbr,openstack-dev/pbr,WiFast/pbr,varunarya10/pbr,citrix-openstack-build/pbr,redhat-openstack/pbr,klmitch/pbr,JioCloud/pbr,citrix-openstack-build/pbr,openstack-dev/pbr,WiFast/pbr,emonty/oslo-packaging,citrix-openstack-build/pbr,klmitch/pbr,emonty/oslo-packaging,redhat-openstack/pbr