Added external_nodes and node_terminus parameters to allow puppet::master to use an ENC

Added external_nodes and node_terminus parameters to allow puppet::master to use an ENC.

According to puppetlabs docs, if external_nodes is specified, the node_terminus setting must be set to 'exec' in order for the ENC to work. Therefore, the module will override any node_terminus parameter passed to be 'exec' only if the external_nodes parameter is also passed.

Wrote exernal_nodes and node_terminus into the two default spec tests parameters. I could write these as separate tests and leave the default tests testing UNSET and the absence of the ini resources in the catalog if it would be helpful. I could also write tests for each combination of external_nodes/node_terminus values to ensure the 'exec' override if that would be helpful. I did not see value of adding this to system:spec since it requires creation of an ENC script.