Add basic variable support

Assignments to arbitrary variables are now supported in Blueprint
files.  Variables cannot be reassigned, and they stay in scope for
the remainder of the current Blueprint file and any subdirs Blueprint
files.  Variables can be used in place of a value in properites or
as an entry in a list.

Change-Id: I04447e4c541304ded802bb82d1ca4ce07d5d95d2