Add management_network_keep Option (#1425)

Starting from version 0.7.0 (https://newreleases.io/project/github/vagrant-libvirt/vagrant-libvirt/release/0.7.0), always_destroy is set to true by default for any network.

However, in certain cases, it would be useful to keep the management network, especially when it has been already configured (externally) with Static IP leases.

This option allows to change the always_destroy behaviour for the management network.

README.md

@@ -978,6 +978,8 @@ used by this network are configurable at the provider level.
 * `management_network_domain` - Domain name assigned to the management network.
 * `management_network_mtu` - MTU size of management network. If not specified,
   the Libvirt default (1500) will be used.
+* `management_network_keep` - Starting from version *0.7.0*, *always_destroy* is set to *true* by default for any network.
+  This option allows to change this behaviour for the management network.
 
 You may wonder how vagrant-libvirt knows the IP address a VM received.  Libvirt
 doesn't provide a standard way to find out the IP address of a running domain.

lib/vagrant-libvirt/config.rb

@@ -62,6 +62,7 @@ module VagrantPlugins
       attr_accessor :management_network_pci_slot
       attr_accessor :management_network_domain
       attr_accessor :management_network_mtu
+      attr_accessor :management_network_keep
 
       # System connection information
       attr_accessor :system_uri
@@ -220,6 +221,7 @@ module VagrantPlugins
         @management_network_pci_bus = UNSET_VALUE
         @management_network_domain = UNSET_VALUE
         @management_network_mtu = UNSET_VALUE
+        @management_network_keep = UNSET_VALUE
 
         # System connection information
         @system_uri      = UNSET_VALUE
@@ -824,6 +826,7 @@ module VagrantPlugins
         @management_network_pci_slot = nil if @management_network_pci_slot == UNSET_VALUE
         @management_network_domain = nil if @management_network_domain == UNSET_VALUE
         @management_network_mtu = nil if @management_network_mtu == UNSET_VALUE
+        @management_network_keep = false if @management_network_keep == UNSET_VALUE
 
         # Domain specific settings.
         @title = '' if @title == UNSET_VALUE

lib/vagrant-libvirt/util/network_util.rb

@@ -32,6 +32,7 @@ module VagrantPlugins
           management_network_pci_slot = env[:machine].provider_config.management_network_pci_slot
           management_network_domain = env[:machine].provider_config.management_network_domain
           management_network_mtu = env[:machine].provider_config.management_network_mtu
+          management_network_keep = env[:machine].provider_config.management_network_keep
           logger.info "Using #{management_network_name} at #{management_network_address} as the management network #{management_network_mode} is the mode"
 
           begin
@@ -92,6 +93,10 @@ module VagrantPlugins
             management_network_options[:slot] = management_network_pci_slot
           end
 
+          if management_network_keep
+            management_network_options[:always_destroy] = false
+          end
+
           # if there is a box and management network is disabled
           # need qemu agent enabled and at least one network that can be accessed
           if (