nuageinit: Ignore non-existent groups

In cloud-init, when a group specified in the 'users.{index}.groups' parameter
does not exist, it is ignored, but the user is created anyway. In the case of
nuageinit, it exits with an exception, since pw(8) expects each group to exist.

Reviewed by: bapt@
Approved by: bapt@
Differential Revision: https://reviews.freebsd.org/D52718

(cherry picked from commit 68691160f41bf6ce9ab70ddeeb7eeec2a7bff245)