Multinode: is_slave bundle variable is a string

By definition in the bundle format, every fields in the attributes array should
be of type string.
We convert 'is_slave' to string to match this requirement.

Change-Id: I083af94c26e324cbf1a282e8a4f982d542802ca4
Multinode: move is_slave in the right section

Change-Id: I4624856342d45471b102a84e3c4c9bc9d5a71f9c
device: Handle empty boot command cases (bug 583).

  * Fix for bug 583 where Target is not handling empty boot
    commands list on preprocessing.
  * Add a general Target object test class to test the
    _boot_cmds_preprocessing method.

Change-Id: I0fcd0a40fdd379c42096a1bd0a8c1552e8fdfb15