speed up standalone installation.

/reviewed-on !96

Ubuntu 18.04 has ansible 2.5. In this version get_md5 changed default
from yes to no (see
https://docs.ansible.com/ansible/latest/modules/stat_module.html)

Warning: this optionwill be removed totally in Ansible 2.9.

/reviewed-on !95

1.0.56 is not buildable because some files are not stored in shacache.
Use master instead. master is buildable most of the time.

This reverts commit 1be62205. It was already fixed in 2fa89eca.

In nexedi/slapos.core!76 and then nexedi/slapos.core!105 we increased the slap proxy table name, so all scripts using `software11` or `partition11` directly break - because table name is different.

/reviewed-on nexedi/slapos.package!77

We need to remove invalid signature certificate for networkcache from current slapos.cfg file because it prevent slapos to download compiled SR from networkcache.

For more information about this problem, see: slapos@04e5d4aa

/reviewed-on !93

OpenVPN fails to build on Debian testing because
net-tools is no longer provided by default.
Depends on nexedi/slapos@67eb9fe4

/reviewed-on nexedi/slapos.package!92

Commit c5e509c3
added the ability to download from network cache when building re6st,
but slapos.libnetworkcache is imported after bin/buildout has
already been generated. The fixup consists of adding libnetworkcache
in bin/buildout from bin/python, which has been renamed to
bin/python.temp until the end of bootstrap to start again if we fail
before the end of the task.

/reviewed-on !90

apt package didn't support direct list input for "name" arg. Use
with_items instead.

the IMT settings already contains slapos master urls

This reverts commit 8fdf3f1a.

It's all the more important as OpenVPN tarballs can't be downloaded with
default Python user-agent.

`slapos.libnetworkcache` depends itself on `setuptools`.

/reviewed-on nexedi/slapos.package!88