Fix allow inbound echo requests + configure SSH keys only when needed

[DeBuXer]

Two small adjustments:

1. Allowing ICMP requests did not work because the variable $fwmgcalPolicy was not defined. As a result, executing it produced the following error message:

[2024-12-05 13:44 +01:00] * Enabling Ping

LocalPolicy        : System.__ComObject
CurrentProfileType : 1

[2024-12-05 13:44 +01:00] - Enable Allow Inbound Echo Requests
The property 'AllowInboundEchoRequest' cannot be found on this object. Verify that the property exists and can be set.
At C:\Program Files (x86)\OpenNebula\context.ps1:916 char:12
+     $ret = $pro.IcmpSettings.AllowInboundEchoRequest = $true
+            ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    + CategoryInfo          : InvalidOperation: (:) [], RuntimeException
    + FullyQualifiedErrorId : PropertyNotFound
The property 'AllowInboundEchoRequest' cannot be found on this object. Verify that the property exists and can be set.
At C:\Program Files (x86)\OpenNebula\context.ps1:916 char:12
+     $ret = $pro.IcmpSettings.AllowInboundEchoRequest = $true
+            ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    + CategoryInfo          : InvalidOperation: (:) [], RuntimeException
    + FullyQualifiedErrorId : PropertyNotFound

[2024-12-05 13:44 +01:00]   ... Failed

2. When the variable $context["SSH_PUBLIC_KEY"] is empty, it is unnecessary to call the functions any further.