Bump libnetwork to 3ac297bc

Bump libnetwork to 3ac297bc7fd0afec9051bbb47024c9bc1d75bf5b in order to
get fix 0c3d9f00 which addresses a flaw that the scalable load balancing
code revealed.  Attempting to print sandbox IDs where the sandbox name
was too short results in a goroutine panic.  This can occur with
sandboxes with names of 1 or 2 characters in the previous code. But due
to naming updates in the scalable load balancing code, it could now
occur for networks whose name was 3 characters and at least one of the
integration tests employed such networks (named 'foo', 'bar' and 'baz').

This update also brings in several changes as well:
 * 6c7c6017 - Fix error handling about bridgeSetup
 * 5ed38221 - Optimize networkDB queue
 * cfa9afdb - ndots: produce error on negative numbers
 * 5586e226 - improve error message for invalid ndots number
 * 449672e5 - Allows to set generic knobs on the Sandbox
 * 6b4c4af7 - do not ignore user-provided "ndots:0" option
 * 843a0e42 - Adjust corner case for reconnect logic

Signed-off-by: Chris Telfer <ctelfer@docker.com>

hack/dockerfile/install/proxy.installer

@@ -3,7 +3,7 @@
 # LIBNETWORK_COMMIT is used to build the docker-userland-proxy binary. When
 # updating the binary version, consider updating github.com/docker/libnetwork
 # in vendor.conf accordingly
-LIBNETWORK_COMMIT=b0186632522c68f4e1222c4f6d7dbe518882024f
+LIBNETWORK_COMMIT=3ac297bc7fd0afec9051bbb47024c9bc1d75bf5b
 
 install_proxy() {
 	case "$1" in

vendor.conf

@@ -37,7 +37,7 @@ github.com/mitchellh/hashstructure 2bca23e0e452137f789efbc8610126fd8b94f73b
 #get libnetwork packages
 
 # When updating, also update LIBNETWORK_COMMIT in hack/dockerfile/install/proxy accordingly
-github.com/docker/libnetwork b0186632522c68f4e1222c4f6d7dbe518882024f
+github.com/docker/libnetwork 3ac297bc7fd0afec9051bbb47024c9bc1d75bf5b
 github.com/docker/go-events 9461782956ad83b30282bf90e31fa6a70c255ba9
 github.com/armon/go-radix e39d623f12e8e41c7b5529e9a9dd67a1e2261f80
 github.com/armon/go-metrics eb0af217e5e9747e41dd5303755356b62d28e3ec