useradd.bbclass: retry useradd/groupadd commands to avoid lock race issues

useradd.bbclass: retry useradd/groupadd commands to avoid lock race issues

A race condition can occur when adding users and groups to the
passwd and group files, causing errors like the following:

 ERROR: Function 'useradd_sysroot' failed
 Tried to access "/etc/group" but this was locked.

This fix will cause the useradd code to retry the useradd and
groupadd commands up to 10 times (with a 1s sleep in between
attempts) before failing.

This fixes [YOCTO #1794]

Signed-off-by: Scott Garman <scott.a.garman@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>