Multi driver support to VPN agent

Add multi driver support to the recipe of vpn_agent,
by changing vpn_device_driver option from string to list.
And add a new option named vpn_device_driver_packages to
install package dependencies according vpn_device_driver.

DocImpact
Closes-Bug: #1386067
Closes-Bug: #1386070
Change-Id: If84827e421a2d94b5ae802a65b3d906670e28e90