support nested variable resolution

[shivdudhani]

fixes #727

Resolve the variables bottom-up (i.e. resolve the inner-most variable to the outer variables). Regular expression leveraged to identify the variables.

There are 2 possible scenarios:

1. No inner variables {{variable}}. The resolved value can be scalar or non-scalar.
2. Contains inner variables {{variable-{{variable}}}}. The inner values need to be scalar for the outer variable can be resolved{{variable-value}}. If the inner variable is non-scalar, the resolution is inconsistent so it's considered as failed variable resolution for reporting.

Limitations:

• Only equal operator is supported for variables
• In the inner-variable resolves to a non-scalar value, then it is considered as a failed variable.