Refactor -o option to simplify code

The change includes the implementation of a click option in print_output
module, being used in ns-list, ns-show, vim-list, vim-show, vnf-list and
vnf-show commands.

It also includes a quick validation that the output format is one of the
allowed outputs. It also fixes jsonpath option so that the output is not
embedded in a [] list structure.

Change-Id: I8c33b5faf341c9a4cdd8d488671f2a81ab3b5a3a
Signed-off-by: garciadeblas <gerardo.garciadeblas@telefonica.com>