uefi_ipxe_bootfile_name config option in /etc/ironic/ironic.conf is missing

ironic-conductor channel: yoga/stable
ironic-conductor revision: 21

`uefi_ipxe_bootfile_name` config option in /etc/ironic/ironic.conf is missing. This makes the DHCP server to advertise the default value of `uefi_ipxe_bootfile_name`, which is `snponly.efi`. See tcpdump below:

```
11:57:29.405148 fa:16:3e:06:9f:aa > ff:ff:ff:ff:ff:ff, ethertype IPv4 (0x0800), length 421: (tos 0xc0, ttl 64, id 28364, offset 0, flags [none], proto UDP (17), length 407)
    10.9.174.64.67 > 255.255.255.255.68: BOOTP/DHCP, Reply, length 379, xid 0xa86b21e1, Flags [Broadcast]
          Your-IP 10.9.174.150
          Server-IP 10.9.174.24
          Client-Ethernet-Address b8:83:03:86:46:70
          Vendor-rfc1048 Extensions
            Magic Cookie 0x63825363
            DHCP-Message Option 53, length 1: ACK
            Server-ID Option 54, length 4: 10.9.174.64
            Lease-Time Option 51, length 4: 86400
            RN Option 58, length 4: 43200
            RB Option 59, length 4: 75600
            Subnet-Mask Option 1, length 4: 255.255.254.0
            BR Option 28, length 4: 10.9.175.255
            Domain-Name Option 15, length 30: "openstack.example.lan"
            Hostname Option 12, length 17: "host-10-9-174-150"
            Domain-Name-Server Option 6, length 8: 10.9.174.64,10.9.174.65
            BF Option 67, length 12: "snponly.efi^@"
            TFTP Option 66, length 12: "10.9.174.24^@"
            Default-Gateway Option 3, length 4: 10.9.174.1
            MTU Option 26, length 2: 1500
```

As `snponly.efi` file is not present in `/tftpboot` directory, the PXE booting process fails with:

```
NBP filename is snponly.efi
NBP filesize is 0 Bytes
PXE-E23: Client received TFTP error from server
```

I observed this problem with `use-ipxe` charm config option set to be `true`.

Workaround:

```
juju run --unit ironic-conductor/0 'echo "uefi_ipxe_bootfile_name=ipxe.efi" >> /var/lib/juju/agents/unit-ironic-conductor-0/charm/templates/parts/section-pxe'

juju run --unit ironic-conductor/0 hooks/config-changed
```