TT#45502 Update Virtualbox VBoxGuestAdditions ISO to v5.2.18

5.2.18 is the latest stable version from VirtualBox,
reported as such by upstream (see
https://download.virtualbox.org/virtualbox/LATEST-STABLE.TXT)
as well as present in current stretch-backports.

Change-Id: Iff5cb0e8ada2419aebbd8b26a507d09d8537ae92

templates/scripts/includes/deployment.sh

@@ -90,8 +90,8 @@ VLAN_SIP_INT=1720
 VLAN_HA_INT=1721
 VLAN_RTP_EXT=1722
 VIRTUALBOX_DIR="/usr/share/virtualbox"
-VIRTUALBOX_ISO="VBoxGuestAdditions_5.1.26.iso"
-VIRTUALBOX_ISO_CHECKSUM="6df8c8ab6e7ac3a70a5e29116f8a5dcdb7dfbd0b226ef849a5cd9502e956b06f" # sha256
+VIRTUALBOX_ISO="VBoxGuestAdditions_5.2.18.iso"
+VIRTUALBOX_ISO_CHECKSUM="f98b6ad7093ee0b27d26dea565b197a5f33fdac93c4b67e73824ce889d6c964c" # 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"