Add support for configuration-based run modes for clush. Run modes are
disabled by default because they rely on external tools that we don't
want to depend on. They can easily be enabled if needed by installing
the corresponding .conf files in clush.conf.d (clush.conf's confdir).

sudo support is provided via an example run mode, by defining two
variables command_prefix and password_prompt:

  [mode:sudo]
  password_prompt: yes
  command_prefix: /usr/bin/sudo -S -p "''"

Then, the sudo mode is activated with --mode or -m:

  $ clush -w nodes -m sudo command

Also added an example of run mode for password-auth based auth with
sshpass.

Main changes:

  clush.conf:
  - add confdir to define paths to config files for run modes
  - add command_prefix
  - add password_prompt
  - removed sudo_command

  clush.conf.d:
  - add sshpass.conf.example
  - add sudo.conf.example

  clush:
  - added --mode
  - removed --sudo

Closes clustershell#198.
Closes clustershell#423.
Fixes clustershell#234.