Fix Icinga Agent configuration from old PowerShell module

[LordHepipud]

Using the old PowerShell module configured the Icinga 2 Agent to write all configuration into one single icinga2.conf. The new framework is using the default config files and will encounter errors if configuration changes apply.

In addition, several Agent Cmdlets will not work properly because features like enable or disable certain features is not supported in this way.

Expected Behavior

The Framework should detect old configurations by the old module and restore the backup for the configuration file.

Current Behavior

In most cases, the configuration for the Icinga 2 Agent will be corrupt

Possible Solution

Add check for old configuration and restore original config

Steps to Reproduce (for bugs)

1. Install the Agent by using the old module
2. Use the new Framework to install/update the agent
3. Change the hostname to FQDN or vice versa to provoke a crash or try enabling/disabling certain features