WIP: bug-fix: prevent buildout to fail when option value is None

When a parameter is None the recipe slapconfiguration fail with the message below:

  2017-09-29 15:51:49 slapos[26200] INFO While:
  2017-09-29 15:51:49 slapos[26200] INFO   Installing.
  2017-09-29 15:51:49 slapos[26200] INFO   Getting section switch_softwaretype.
  2017-09-29 15:51:49 slapos[26200] INFO   Initializing section switch_softwaretype.
  2017-09-29 15:51:49 slapos[26200] INFO   Getting option switch_softwaretype:default.
  2017-09-29 15:51:49 slapos[26200] INFO   Getting option switch_softwaretype:kvm.
  2017-09-29 15:51:49 slapos[26200] INFO   Getting section dynamic-template-kvm.
  2017-09-29 15:51:49 slapos[26200] INFO   Initializing section dynamic-template-kvm.
  2017-09-29 15:51:49 slapos[26200] INFO   Getting section slap-configuration.
  2017-09-29 15:51:49 slapos[26200] INFO   Initializing section slap-configuration.
  2017-09-29 15:51:49 slapos[26200] INFO Error: Missing option: slap-configuration:tun

I realized that when a parameter (which has default value to '') is not set by the user while requesting instance, it become None then adding options[key] = None in slapconfiguration will make buildout angry. If I print the result of options at the end, all keys with values equal to None are missing.

I general If do this:

     [request-xxxx]
     .....
     config-foo-bar = {{ dumps(kvm_parameter_dict.get('foo-bar', '')) }}

or

    [param-fooo]
    bar = 
    
     [request-xxxx]
     .....
     config-foo-bar = ${param-fooo:bar}

Buildout will raise with exception saying Error: Missing option: slap-configuration:foo-bar. Setting the value to empty string instead of None solve the problem.