Reserve IP address for out of bounds machines

Ability to define IP address beside of choosing VMs in Static NAT, Port Forward and Load balancer services to provide those services to servers outside of CloudStack.
##### ISSUE TYPE

* Feature Idea

##### COMPONENT NAME

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VR
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##### CLOUDSTACK VERSION

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All versions
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##### CONFIGURATION

Advanced networking

##### OS / ENVIRONMENT

N/A

##### SUMMARY

In some network services we need to specify which VM should use it. For example Static NAT, Port Forward and Load balancer.
It would be a good idea if we can also specify IP address of a server which is not managed by CloudStack.
This feature helps us to use network services provided by virtual router for physical servers or even VMs outside of CloudStack management area but share a common L2 network with other VMs in CloudStack.
This feature also requires reserve IP address capability in all type of networks. We can expand the issue #4357 for private IPs in isolated networks too.

##### STEPS TO REPRODUCE

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1. Create an isolated network
2. In Port forward section define a new rule
3. In "Add VM" panel write IP address of the server outside of CloudStack

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##### EXPECTED RESULTS

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An iptables rule created in virtual router to forward desired port to the server IP

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##### ACTUAL RESULTS

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It is not possible to mention IP address of the server outside of CloudStack

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Beginnen Sie mit den CloudStack-Implementierungen des virtuellen Routers für Static NAT, Portweiterleitung und Load Balancing, und prüfen Sie Issue #4357 hinsichtlich der damit verbundenen Arbeiten zur Reservierung privater IPs. Als erledigt gilt dies, wenn diese Dienste eine IP außerhalb der CloudStack-Verwaltung ansteuern können und der virtuelle Router die entsprechende Weiterleitungsregel über alle unterstützten Netzwerktypen hinweg erstellt.

Vom Indexierungsmodell aus dem Issue-Text verfasst.