move cli check_*() args checking functions

Functions check_disks(), check_networks() and check_event() of module
CloubedCli are now methods of CloubedArgumentParser class. At the same
time, their names have changed from check_*() to parse_*() for
homogeneity with other method of this class.

The logic to check the values of these parameters against the content of
cloubed YAML file has been removed because:
1/ it didn't work.
2/ it shouldn't be better be done in lib/Domain.py. TODOs have been
   added at appropriate lines of this line for future patches.
updated logic to check coherency of optional args

The logic in the method CloubedArgumentParser.check_optionals() has
been significantly evolved (see comments for details). Arguments --event
and --filename have been set as required for respectively wait and gen
actions.

Others small changes:
- Raise exception if --bootdev parameter has no value
- Added missing docstring to methods.
add new unit tests for CloubedArgumentParser class

Fixes #17.