Add helper to create/remove/update user with easy-rsa support

[serhepopovych]

This should include steps from README

$ sudo useradd -c 'PKI certificate management user' -m certmgr
$ sudo -u certmgr -i cp -a /opt/easy-rsa/skel/easy-rsa ~certmgr
$ sudo -u certmgr -i chown -R certmgr:certmgr ~certmgr/easy-rsa

$ sudo ln -sf ~certmgr/easy-rsa/.htconf
/etc/lighttpd/vhosts.d/easy-rsa.example.net/users.d/certmgr
#

and will help managing users with easy-rsa access.