Added support for vpn gateway connection distribute traffic

[uibm]

Distributing traffic across tunnels of route-based VPN gateway connections. You can now distribute traffic across tunnels with a status of up in a route-based VPN gateway connection. When creating or updating a route-based VPN gateway connection, set the distribute_traffic property to true (default is false). Existing connections will have the distribute_traffic property set to false

Changes in datasource :

• vpn_gateway_connection

  • code
  • test
  • doc

• vpn_gateway_connections

  • code
  • test
  • doc

Changes in resource :

• vpn_gateway_connection
  • code
  • test
  • doc

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Relates OR Closes #0000

Output from acceptance testing:

$ make testacc TESTARGS='-run=TestAccXXX'

...