conman:  Fix slow connection checks.

1) Don't run connection checks without a default route.

2) When getting a new gateway, check for a new subnet.  This means
  routes get added in the right order the first time around, instead
  of failing and getting fixed some time later.

Change-Id: Id15f2d027c1c45caae139ce90ed8cdf48b419d74
2 files changed