Overriding parameters in a parameter set still requires the overriden parameter to be resolvable

[kichristensen]

Describe the bug

When overriden a parameter specified in a parameter set, the overriden parameter should not be resolved as specified in the parameter set.
This block scenarios, where you override a parameter read from an environment variable that haven't been defined, because it is overriden.

According to the documentation:

When a bundle is executed, parameter values are resolved in a hierarchy of (from highest to lowest precedence):

• Overrides specified either with the --param flag or by setting parameters directly on an installation resource
• Parameter sets specified with the --parameter-set flag or by setting the parameter set name directly on an installation resource
• Parameter values remembered from the last time the bundle was run
• Defaults defined for the parameter in the bundle definition

To Reproduce

Steps to reproduce the behavior:

1. Create and apply a parameter set containing the value name with an environment source, e.g.:

{
    "schemaType": "ParameterSet",
    "schemaVersion": "1.0.1",
    "name": "hello-llama",
    "parameters": [
        {
            "name": "name",
            "source": {
                "env": "FOO"
            }
        }
    ]
}

2. Run porter install --reference getporter/hello-llama:v0.1.1 -p hello-llama --param name=Bar
3. See error describing that the parameter hello-llama.name could not be resolved

Expected behavior

The installation should succeed because the parameter is overriden, and should not be read from the parameter set.

Porter Command and Output

$ porter install --reference getporter/hello-llama:v0.1.1 -p hello-llama --param name=Bar
unable to process provided parameter sets: 1 error occurred:
        * unable to resolve parameter hello-llama.name from env FOO: environment variable env is not defined

Version

porter v1.2.1 (aa97c59)