Pass the proper admin context to update_dhcp

Bug 1180111

Commit 14e2ed3ace769c4deaca3c48a2fa59863f637a5f introduced changes
to contexts passed around during resize. One of the calls to
update_dhcp was missed and was causing stack traces on
resize-revert.

Change-Id: Ib7bec51324c7bced009407af13a7fff7a724e020
(cherry picked from commit 42cc18da31ceb10ec0b26fe734e7556873f6fe71)
Merge "Pass the proper admin context to update_dhcp" into stable/grizzly