json_parser_utils: Add variable support

Enhance the json_parser_utils functions to support optional usage of
variables in JSON values.

Variables are specified using the syntax `${variable_name}`.

json_parser_utils: Add variable support

Enhance the json_parser_utils functions to support optional usage of
variables in JSON values.

Variables are specified using the syntax `${variable_name}`.

Variable values are specified in an optional new parameter to the
parsing functions. Parsing functions will replace the variable with the
corresponding variable value.

Example:
```
{
"inventory_path": "/xyz/openbmc_project/inventory/system/chassis${chassis_number}"
}
```

Tested:
* Ran automated test cases.

Change-Id: Ib8f5d9b27ccc96ca9d16eb9a044321233f81ba18
Signed-off-by: Shawn McCarney <shawnmm@us.ibm.com>

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pseq: Create Chassis class

Create a C++ Chassis class that corresponds to the JSON "chassis" object
in the config file.

Eventually more function will be added to this class, but for the
initial ver

pseq: Create Chassis class

Create a C++ Chassis class that corresponds to the JSON "chassis" object
in the config file.

Eventually more function will be added to this class, but for the
initial version only implement the required support for the JSON
properties.

Create automated test cases for the new class.

Tested:
* Ran automated test cases.

Change-Id: I66bd795f894b3376a5cb65294db7dbb6cf37e891
Signed-off-by: Shawn McCarney <shawnmm@us.ibm.com>

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