TT#35952 Update VBoxGuestAdditions from 5.2.18 to 5.2.26

The version 5.2.18 building is very slow on the recent
Debian stretch kernel 4.9.0-8-amd64:

VBoxGuestAdditions version 5.2.18:

> 12:32:52 (netscript.grml:1854): vagrant_configuration():  grml-chroot /mnt /media/cdrom/VBoxLinuxAdditions.run --nox11
> Writing /etc/debian_chroot ...
> Verifying archive integrity... All good.
> Uncompressing VirtualBox 5.2.18 Guest Additions for Linux........
> VirtualBox Guest Additions installer
> Copying additional installer modules ...
> Installing additional modules ...
> VirtualBox Guest Additions: Building the VirtualBox Guest Additions kernel modules.  This may take a while.
> VirtualBox Guest Additions: Starting.
> VirtualBox Guest Additions: Building the VirtualBox Guest Additions kernel modules.  This may take a while.
> +12:39:15 (netscript.grml:1856): vagrant_configuration(): ...

VBoxGuestAdditions version 5.2.26:

> +13:35:50 (netscript.grml:1854): vagrant_configuration():  grml-chroot /mnt /media/cdrom/VBoxLinuxAdditions.run --nox11
> Writing /etc/debian_chroot ...
> Verifying archive integrity... All good.
> Uncompressing VirtualBox 5.2.26 Guest Additions for Linux........
> VirtualBox Guest Additions installer
> Copying additional installer modules ...
> Installing additional modules ...
> VirtualBox Guest Additions: Building the VirtualBox Guest Additions kernel
> modules.  This may take a while.
> VirtualBox Guest Additions: To build modules for other installed kernels, run
> VirtualBox Guest Additions:   /sbin/rcvboxadd quicksetup <version>
> VirtualBox Guest Additions: Building the modules for kernel 4.9.0-8-amd64.
> VirtualBox Guest Additions: Starting.
> Running in chroot, ignoring request: daemon-reload
> +13:39:12 (netscript.grml:1856): vagrant_configuration():

5.2.26 is the latest stable version from VirtualBox,
reported as such by upstream (see
https://download.virtualbox.org/virtualbox/LATEST-STABLE.TXT)

Change-Id: Ieb4b158344b3e4d0bf2719e8897cdfcdf133082b

templates/scripts/includes/deployment.sh

@@ -88,8 +88,8 @@ VLAN_SIP_INT=1720
 VLAN_HA_INT=1721
 VLAN_RTP_EXT=1722
 VIRTUALBOX_DIR="/usr/share/virtualbox"
-VIRTUALBOX_ISO="VBoxGuestAdditions_5.2.18.iso"
-VIRTUALBOX_ISO_CHECKSUM="f98b6ad7093ee0b27d26dea565b197a5f33fdac93c4b67e73824ce889d6c964c" # sha256
+VIRTUALBOX_ISO="VBoxGuestAdditions_5.2.26.iso"
+VIRTUALBOX_ISO_CHECKSUM="b927c5d0d4c97a9da2522daad41fe96b616ed06bfb0c883f9c42aad2244f7c38" # sha256
 VIRTUALBOX_ISO_URL_PATH="/files/${VIRTUALBOX_ISO}"
 SIPWISE_APT_KEY_CHECKSUM="f4cdbe4994ae8ca6c4b24eb164e82a20579b335da4eca0907ecaace832e9a0a7" # sha256
 SIPWISE_APT_KEY_PATH="/etc/apt/trusted.gpg.d/sipwise.gpg"