Add adoption methods for adding items into a PartitionVector (#759726)

The current code uses push_back() and insert() to copy Partition objects
into the vector of pointers.  This has a few issues:
1) Unnecessary copying of Partition objects;
2) Hides the nature of the PartitionVector class as a manager of
   pointers to Partition objects by providing copy semantics to add
   items.  It is generally better to be explicit;
3) C++ doesn't provide polymorphic copy construction directly, but this
   is easily worked around by following the Virtual Constructor idiom
   [1], which would allow PartitionLUKS derived class objects to be
   copied into the vector.

Add push_back_adopt() and insert_adopt() methods which add a pointer to
a Partition object into the PartitionVector adopting ownership.

[1] Wikibooks: More C++ Idioms / Virtual Constructor
    https://en.wikibooks.org/wiki/More_C%2B%2B_Idioms/Virtual_Constructor

Bug 759726 - Implement Partition object polymorphism