cli: implement validate functionality

[audrium]

Usage: reana-client validate [OPTIONS]

  Validate workflow specification file.

  The ``validate`` command allows to check syntax and validate the reana.yaml
  workflow specification file.

  Examples:

       $ reana-client validate -f reana.yaml

Options:
  -f, --file PATH          REANA specification file describing the workflow to
                           execute. [default=reana.yaml]
  --environments           If set, check all runtime environments specified in
                           REANA specification file. [default=False]
  --pull                   If set, try to pull remote environment image from
                           registry to perform validation locally. Requires
                           ``--environments`` flag. [default=False]
  --server-capabilities    If set, check the server capabilities such as
                           workspace validation. [default=False]
  -t, --access-token TEXT  Access token of the current user.
  --help                   Show this message and exit.

Note that it requires loading and validating the specification file. It could be tackled later once we make the clients thin and refactor the spec upload logic to send the whole yaml file to the server side and process it there.