[PDF] Fix yamllint issues in installer adapters

[PDF] Fix yamllint issues in installer adapters

Although current YAML files in Pharos are all valid according to
yamllint, the resulting YAMLs after PDF/IDF + IA interpolations
still have a lot of violations, which need to be addressed.

PDF changes:
- bii/pod1:
  + fix typo in disks_A reference;

IDF changes:
- replace empty strings with `~` keyword (will be expanded to 'None');

IA (installer adapter) changes:
- apex:
  + add missing document start;
- compass4nfv:
  + too few spaces before comment;
  + missing starting space in comment;
  + replace '\t' with spaces;
- daisy:
  + None;
- fuel:
  + check conf.idf is defined first (fix parse for PODs without IDF);
  + move document start outside conf.idf condition
- joid:
  + None;

This is not an exhaustive change, some yamllint issues are still there
but require either refactoring IAs or changing the PDF/IDF files in a
way that needs the respective maintainer's input.

Change-Id: I26743e265217e892b6a94de96a016c295ea24fb5
Signed-off-by: Alexandru Avadanii <Alexandru.Avadanii@enea.com>