$ sudo ls /etc/netplan/
01-network.yaml         50-cloud-init.yaml.bak

$ sudo cat /etc/netplan/01-network.yaml
network:
  version: 2
  renderer: networkd
  ethernets:
    enp0s3:
      dhcp4: yes
    enp0s8:
      addresses: [192.168.56.2/24]
      gateway4: 192.168.56.1
      dhcp4: no

$ sudo cat /etc/netplan/50-cloud-init.yaml.bak
# This file is generated from information provided by
# the datasource.  Changes to it will not persist across an instance.
# To disable cloud-init's network configuration capabilities, write a file
# /etc/cloud/cloud.cfg.d/99-disable-network-config.cfg with the following:
# network: {config: disabled}
network:
    version: 2
    ethernets:
        enp0s3:
            dhcp4: true

$ sudo cat /etc/cloud/cloud.cfg.d/99-disable-network-config.cfg
network: {config: disabled}

$ sudo ifconfig -a
enp0s3: flags=4163<UP,BROADCAST,RUNNING,MULTICAST>  mtu 1500
        inet 10.0.2.15  netmask 255.255.255.0  broadcast 10.0.2.255
        inet6 fe80::a00:27ff:fe0e:f506  prefixlen 64  scopeid 0x20<link>
        ether 08:00:27:0e:f5:06  txqueuelen 1000  (Ethernet)
        RX packets 2  bytes 1180 (1.1 KB)
        RX errors 0  dropped 0  overruns 0  frame 0
        TX packets 15  bytes 1704 (1.7 KB)
        TX errors 0  dropped 0 overruns 0  carrier 0  collisions 0

enp0s8: flags=4163<UP,BROADCAST,RUNNING,MULTICAST>  mtu 1500
        inet 192.168.56.2  netmask 255.255.255.0  broadcast 192.168.56.255
        inet6 fe80::a00:27ff:febe:a750  prefixlen 64  scopeid 0x20<link>
        ether 08:00:27:be:a7:50  txqueuelen 1000  (Ethernet)
        RX packets 508  bytes 47165 (47.1 KB)
        RX errors 0  dropped 0  overruns 0  frame 0
        TX packets 462  bytes 53569 (53.5 KB)
        TX errors 0  dropped 0 overruns 0  carrier 0  collisions 0

lo: flags=73<UP,LOOPBACK,RUNNING>  mtu 65536
        inet 127.0.0.1  netmask 255.0.0.0
        inet6 ::1  prefixlen 128  scopeid 0x10<host>
        loop  txqueuelen 1000  (Local Loopback)
        RX packets 6  bytes 402 (402.0 B)
        RX errors 0  dropped 0  overruns 0  frame 0
        TX packets 6  bytes 402 (402.0 B)
        TX errors 0  dropped 0 overruns 0  carrier 0  collisions 0

# apt-get install mysql-server

E: Sub-process /usr/bin/dpkg returned an error code (1)

root@dev:~# apt-get install mysql-server
Reading package lists... Done
Building dependency tree       
Reading state information... Done
The following extra packages will be installed:
  mysql-server-5.5
Suggested packages:
  tinyca
The following NEW packages will be installed:
  mysql-server mysql-server-5.5
0 upgraded, 2 newly installed, 0 to remove and 0 not upgraded.
Need to get 0 B/1,846 kB of archives.
After this operation, 32.5 MB of additional disk space will be used.
Do you want to continue? [Y/n] y
Preconfiguring packages ...
(Reading database ... 48232 files and directories currently installed.)
Preparing to unpack .../mysql-server-5.5_5.5.54-0+deb8u1_amd64.deb ...
Aborting downgrade from (at least) 10.1 to 5.5.
If are sure you want to downgrade to 5.5, remove the file
/var/lib/mysql/debian-*.flag and try installing again.
dpkg: error processing archive /var/cache/apt/archives/mysql-server-5.5_5.5.54-0+deb8u1_amd64.deb (--unpack):
 subprocess new pre-installation script returned error exit status 1
Selecting previously unselected package mysql-server.
Preparing to unpack .../mysql-server_5.5.54-0+deb8u1_all.deb ...
Unpacking mysql-server (5.5.54-0+deb8u1) ...
Errors were encountered while processing:
 /var/cache/apt/archives/mysql-server-5.5_5.5.54-0+deb8u1_amd64.deb
E: Sub-process /usr/bin/dpkg returned an error code (1)