fips: add pre_enable prompt indicating that additional debs required

[blackboxsw]

Addresses the last remaining FIPS-related prompt and message when
running: ua enable fips.

Fixes: #1031

To test:

cat > ua-dev.yaml <<EOF
#cloud-config
packages:
 - git
 - make
runcmd:
 - git clone -b feature/fips-pre-enable-prompt https://github.com/blackboxsw/ubuntu-advantage-client.git /var/tmp/uac
 - cd /var/tmp/uac/
 - make deps
 - dpkg-buildpackage -us -uc
 - apt-get remove ubuntu-advantage-tools   # Issue: #1044
 - dpkg -i /var/tmp/ubuntu-advantage-tools_20.4_amd64.deb
 - ua attach <YOUR_TOKEN>
EOF

multipass launch daily:xenial --cloud-init ua-dev.yaml  -n dev-xx
# connect to the vm and test fips prompts
multipass connect dev-xx
cloud-init status --wait --long
sudo ua status
sudo ua enable fips

[patriciadomin]

Many thanks, +1