Use safe JSON serialisation/deserialisation, as otherwise unusual
slave_references can lead to issues and also character case is not kept.

Also care about case of log access user, which was undetected since
slave_reference in tests were always lowercase.

As some of the nodes can lag behind, the system can be in state, that those
nodes will send inactive (also destroyed) slave publish information. Before
publishing it to master, check if each of slaves is really present on master.

Tasks:

 - [x] prove it really works on simulated environment
 - [x] check impact on massive simulated environment
 - [x] cover with a test (optionally)
 - [ ] check test results with this change

/reviewed-on nexedi/slapos!519

/cc @tomo @jm

/reviewed-on nexedi/slapos!520

OPENSSL_BINARY is not needed in the test case, only SQLBENCH_PATH has to be
provided, and perl shall be available in the PATH.

Notes

stack/erp5

 * The service-auto-approve-amount to default is set to 1, in order that the
   only needed service is automatically approved.  As caucase is accessed
   internally (on local IPv4) only partitions on the same server will access
   caucase.

/reviewed-on nexedi/slapos!518

  This commit add srsLTE into slapos. More informations about
  srsLTE at: https://github.com/srsLTE/srsLTE

  This still a proof of concept because:

   - Not all dependencies are ported into slapos
   - The configuration files are not turned into parameters
   - enb and epc are running on the same partitions
   - Usage of sudo is a code crime, but it is the only way currently**

** This would require patch the srsLTE, to allow external tools pre-configure
  the environment. ie.: Create network interface.

Use json.dumps/.loads to generate promise plugin code. This is safe as it will correctly escape sring and prevent code injection from untrustable parameters.

Add 'import' parameter to simplify parameters used to generate the script instead of passing full import code. When import parameter is set, promise will be loaded from that import path and parameter `content` is ignored.

/reviewed-on !515

Close the NBD socket when testing it so it won't mess up qemu.

/reviewed-on nexedi/slapos!516

- Add `buildout.hash.cfg` file so this way we can use the useful `update-hash` script
- Refactor a bit the `software.cfg` file sections as it was repeating itself

/reviewed-on nexedi/slapos!512

Extend the backend with Timeout configuration via headers and use it
to prove, that request taking more than try_duration is correctly served.

Also prove that try_duration and try_interval are correct passed to the slave
configuration.