Updates to Chocolatey Ensure Installed, Chocolatey Install Package, SQL - Backup Database

Chocolately changes: Changed how the environment variable for the chocolatey install
directory was retrieved. Using the $env: syntax won't find an
environment variable that was just added, it only pulls variables that
existed when the process was created. Calling the Get-EnvironmentVariable
method queries the registry directly.

This lets you install and use chocolatey without having to restart the
Octopus tentacle or the machine after the installation. This is also a
handy way to check environment variables thate may have been created
after the process you're running from was spawned.

SQL Backup Changes:
I also fixed a bug in the SQL Backup step template, and added support for specifying SQL Compression option for the backup code.