Add sleep command to be sure that network is properly initialized in rescue system

Hi, 
After the backup of a server with the eth0 interface configured in DHCP mode. 
I started the recovery system, and the dhclient command has not been executed. The culprit is get_device_by_hwaddr that returns no interface. I think this command was executed too early because when I sourced manually 58-start-dhclient.sh the script works fine ! Adding a sleep 5 before calling get_device_by_hwaddr fixes this issue (tested 5 or 6 times to be sure )

```bash
RESCUE # cat /etc/rear/rescue.conf
# line below was automatically added by 21_include_dhclient.sh
USE_DHCLIENT=y
DHCLIENT_BIN=dhclient
DHCLIENT6_BIN=
NETFS_KEEP_OLD_BACKUP_COPY=""
NETFS_PREFIX="kbac11"
```