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 System class

Create a C++ System class that corresponds to the entire system being
controlled by the BMC.

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

pseq: Create System class

Create a C++ System class that corresponds to the entire system being
controlled by the BMC.

Eventually more function will be added to this class, but for the
initial version only implement the required support for storing the
chassis objects from the JSON config file.

Create automated test cases for the new class.

Tested:
* Ran automated test cases.

Change-Id: I90b05acb3d17558b4b518c88b45d3d5825165a2a
Signed-off-by: Shawn McCarney <shawnmm@us.ibm.com>

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