Support NPM installation switch

[ghost]

Tested via local change:
default['datadog']['windows_agent_url'] = 'https://s3.amazonaws.com/dd-agent-mstesting/builds/tagged/'
default['datadog']['windows_npm_install'] = true
default['datadog']['system_probe']['enabled'] = true
dd_agent_installer = "datadog-agent-7.27.0-rc.4-1-x86_64.msi"

and running command:
kitchen converge dd-agent-windows-2012r2-13113

Observed the VM provisioned properly, NPM installed and "C:/ProgramData/Datadog/system-probe.yaml" is created.

[albertvaka]

LGTM, but I think we can improve the user experience:

NPM only runs if system-probe.yaml contains a section like this:

network_config:
    enabled: true

Although it's technically possible to use Windows NPM with the current PR, you need to do three things:

• Setting windows_npm_install = true to install the driver
• Setting system_probe.enabled = true to create the system-probe.yaml file
• Setting system_probe.extra_config: {"network_config": { "enabled": true } } to write the above section in the file.

We can simplify this to a great extent with the addition of a new chef property:

default['datadog']['system_probe']['network_enabled'] = true

This property could have the three effects mentioned: installing the driver, enabling the system-probe.yaml creation and writing the above section to the file. We can maintain the existing properties (eg: windows_npm_install) for users who want full control, but for most cases being able to set a single property will simplify the user experience.

[ghost]

Found and fixed some issues:

1. Sysprobe recipe only worked on Linux with agent version > 6.11. This has been now fixed to allow it work on Windows with agent version > 7.26. In the first commit, system_probe.yaml appeared during test, but it was from the MSI.
2. system_probe.yaml.erb loads values directly (except for extra config), so the code in system-probe.rb setting the config file never worked. This has been fixed to load values from ruby file.

Tested via local changes below:
default['datadog']['agent_version'] = '7.27.0'
default['datadog']['windows_agent_url'] = 'https://s3.amazonaws.com/dd-agent-mstesting/builds/tagged/'
default['datadog']['system_probe']['network_enabled'] = true
dd_agent_installer = "datadog-agent-7.27.0-rc.4-1-x86_64.msi"

The comment has been addressed in the latest commit.

[albertvaka]

Left one comment but otherwise LGTM.