Openssh (sshd) doesn't want to start with keys generated by dropbear

slaprunner: updates export script to make sure that etc directory exists in backup before copying into it

When it is restored on importer instance, it erases its RSA key... Which makes the sshd service to crash

slaprunner: uses sshkeys_authority recipe to manage openssh keys, otherwise they may change at any slapos run

This makes easier the development of the webrunner, as it can be deployed within a webrunner

Changes include:
  * new parameter "no-ipv4-frontend" to prevent the request of an ipv4 frontend. The webrunner cannot provide one, so let's get rid of failing promises.
  * new paramaters to define the ports of the ssh servers (for runner-importer and runner-exporter). Then no colliding when servers run on the same IP.

Revert "slaprunner: fix resiliency test, add development-mode (bool) parameter for easy build webrunner in webrunner"

This reverts commit 59966611.

hosts-dict values cannot be domain names.
memcached entries are implementation-independent (no mention of kumofs
anymore).
relational database information (which does not follow above memcached
rule, sadly, naming mariadb explicitly) is published in two lists.
Add minimal patterns to document what requester should expect.
Document deadlock-debugger-password".
Drop non-standard "optional" property from jupyter-url declaration.

slaprunner: fix resiliency test, add development-mode (bool) parameter for easy build webrunner in webrunner

This is backported to ZODB-3.10 https://github.com/zopefoundation/persistent/pull/44

The patch is needed for wendelin.core to being able to relase not needed
memory.

/reviewed-on !93