Separate pool parsing from pool auto-allocation
Separate pool parsing from pool auto-allocation

- In default ipam driver. Also improve error message
  in case no good pool is found

Signed-off-by: Alessandro Boch <5bd2eb5b6f60921d9a2b9cb6a0a86bb947f3d361@docker.com>

mahak/libnetwork,abhi/libnetwork,abhi/libnetwork,sainath14/libnetwork,mahak/libnetwork,curtiszimmerman/libnetwork,tmakita/libnetwork,allencloud/libnetwork,mYmNeo/libnetwork,vdemeester/libnetwork,medallia/libnetwork,docker/libnetwork,allencloud/libnetwork,mYmNeo/libnetwork,docker/libnetwork,mahak/libnetwork,fcrisciani/libnetwork,curtiszimmerman/libnetwork,ityangchen/libnetwork,abhi/libnetwork,mbanikazemi/libnetwork,msabansal/libnetwork,ityangchen/libnetwork,WeiZhang555/libnetwork,kolyshkin/libnetwork,vdemeester/libnetwork,kolyshkin/libnetwork,medallia/libnetwork,tmakita/libnetwork,docker/libnetwork,fcrisciani/libnetwork,sainath14/libnetwork,vdemeester/libnetwork,medallia/libnetwork,msabansal/libnetwork,mbanikazemi/libnetwork,kolyshkin/libnetwork,WeiZhang555/libnetwork