Allow powershell install files inside a broker type

Windows machines, being unable to run Bash files, will need an alternative
file format for each broker. This had two options: either make new broker
types for windows variants (`puppet-windows`, `noop-windows` etc.) or allow
various file extensions inside the same broker.

This commit chooses the latter. In order for a Windows machine to select a
powershell script, the task needs a piece of metadata called
`broker_script_type` that equals 'powershell'. This will search for an
`install.ps1.erb` file inside the broker when the node is looking for its
broker script.

Right now, only 'bash', blank (defaulting to 'bash'), and 'powershell' are
allowed. This list is maintained in code (`broker_type.rb`).

Fixes https://tickets.puppetlabs.com/browse/RAZOR-355