Support File-based parameters

[kmanning]

• Some teams simply aren't ready to start storing parameters in ParameterStore
• ^-- that will likely prevent some teams from using this pipeline until they're ready to use ParameterStore (if ever)
• Provide the ability to store key/value pairs directly in a file in their project
• The files should be named after the environment by default

[kmanning]

Eg:

FileBasedParameters.init()

def validate = new TerraformValidateStage()

// Looks for a qa.properties file, loads contents, and breaks into key/value pairs based on newline
def deployQa = new TerraformEnvironmentStage('qa')

// Looks for a uat.properties file, loads contents, and breaks into key/value pairs based on newline
def deployUat = new TerraformEnvironmentStage('uat')

validate.then(deployQa)
        .then(deployUat)
        .build()

[kmanning]

Teams should explicitly be discouraged from storing sensitive data in the properties files. Reference users to CredentialsPlugin, Issue #67 .