(feat) Allow nesting

[ranjib]

Allow running full blown blender scripts as a task from other blender jobs.

• Introduce blend_task as DSL method to specify sub-blends :-)
• the new task allows specifying strategy, concurrency, script path etc

blend_task 'cassandra_repair' do
  script 'blends/repair.rb'
  strategy :per_host
end

By default, driver specific configurations are not passed. User can either specify configurations explicitly, or tell blender to copy a list of driver configurations.

Example of specifying explicit configurations:

blend_task 'cassandra_repair' do
  script 'blends/repair.rb'
  config :ssh, user: 'ubuntu', password: 'test'
end

To copy over the configurations from current blender script:

blend_task 'cassandra_repair' do
  script 'blends/repair.rb'
  pass_configs :ssh
end