Packaging improvements

[0pcom]

Did you run make format && make check?
no
Fixes #

• autoconfiguration of visor and hypervisor upon package installation

Changes:

• generates ssl key and cert and adds them to the package at the location they are found at in skybian following the procedure detailed here
• adds postinst script which is automatically executed upon package instalation to accomplish configuration of hypervisor and visor.
• adds package-based method of setting / distributing the hypervisor public key
• adds local apt repository configuration for distributing the hypervisorconfig package
• hosts the apt repo on the local network with readonlycache

Notes:

• the hypervisor configuration does not take place if the hypervisorconfig package is installed
• the hypervsorconfig package is created in the postinst script when the hypervisor is configured, but it is not installed on the hypervisor. It is added to the hypervisor's apt repo in /var/cache/apt/repo
• the configuration of the apt repo hosted by the hypervisor in the visor's /etc/apt/sources.list is not included in these scripts.

This should be done either manually by the user in a ssh session or via a functionality which can be added to skyimager. Example:

add-apt-repository 'deb  [trusted=yes] http://192.168.0.2 sid main'

Substituiting in the above command the debian codename which is listed in /etc/os-release

Issues:

• the visor fails to start when the hypervisor key is set in /etc/skywire-visor.json

failed to parse config. error="json: cannot unmarshal object in go struct field V1.hypervisors of type cypher.Pubkey"

This does not happen with the master branch, only with develop.

Conflicts:
The extent of the configuration which is accomplished by the postinst script may cause difficulty when it is installed in a chroot in skybian.

How to test this PR:

make package-amd64
dpkg -i *.deb

go to 127.0.0.1:8000

hypervisor is running, visor would be running if not for the aforementioned errors.