shill: DHCPConfig: Get new lease with RenewIP

If a DHCP process is not running when RenewIP is called, acquire
a new lease instead of returning.  This allows callers external
to shill to avoid having to keep track of whether the IPConfig
object has a lease or not.

BUG=chromium:318290
TEST=Unit test + manual: Reproduce the issue in crbug.com/318290.
In specific, change the IP address to a static IP first and close
the configuration panel, then separately switch to custom name
servers.

Originally-Reviewed-on: https://chromium-review.googlesource.com/177350
(cherry picked from commit abb99ef706a8d6895c5137a5b42a0703fe3fec12)

Change-Id: I07bc86b893a502a7fceb86972b04e87928644cd1
Reviewed-on: https://chromium-review.googlesource.com/177762
Reviewed-by: Paul Stewart <pstew@chromium.org>
Commit-Queue: Paul Stewart <pstew@chromium.org>
Tested-by: Paul Stewart <pstew@chromium.org>