Rename for clarity

Signed-off-by: Maria Shaldibina <mariash@pivotallabs.com>
Simplify Container#spawn

  * remove arguments which never change
  * simplify our use of protocol class

Signed-off-by: Maria Shaldibina <mariash@pivotallabs.com>
Move ResourceLimit creation to its own method

Signed-off-by: Maria Shaldibina <mariash@pivotallabs.com>
Use new Container#resource_limits method to simplify #spawn

Signed-off-by: Maria Shaldibina <mariash@pivotallabs.com>
Allow ResourceLimits to be optional when calling #spawn

  * this enables us to use #spawn for staging tasks

Signed-off-by: Maria Shaldibina <mariash@pivotallabs.com>
Fix quotes and spacing

Signed-off-by: Maria Shaldibina <mariash@pivotallabs.com>