control: Add Modifier utility

Create a Modifier utility class that provides a doOp() function that can
modify a value based on the JSON config passed to its constructor.

With the JSON:
 {
   "operator": "minus",
   "value": 3
 }

 doOp() will subtract 3 from the passed in value.

 The only currently supported expression is 'minus'.

 This will be used by future actions to read a D-Bus property, subtract
 some configurable value from it, and store it in the Manager's
 parameter store.

Signed-off-by: Matt Spinler <spinler@us.ibm.com>
Change-Id: I31d9800f83e52a927a2061a9a896da9c19d35809
3 files changed