webmin: fix QA host-user-contaminated

Due to the way most files were installed, using cp ..., during packaging we got spammed
with messages like:

WARNING: QA Issue: webmin: /webmin-module-fail2ban/usr/lib/webmin/webmin/fail2ban/lang/no is owned by gid 100, which is the same as the user running bitbake. This may be due to host contamination [host-user-contaminated]
WARNING: QA Issue: webmin: /webmin-module-system-status/usr/lib/webmin/webmin/system-status/lang/no is owned by gid 100, which is the same as the user running bitbake. This may be due to host contamination [host-user-contaminated]

Do the install in a similar way as is done in bin_package.bbclass.
By doing that, we're not getting any QA-errors from host-user-contaminated.

Signed-off-by: Anders Darander <anders@chargestorm.se>
Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>