Added config variable for External Network type in ML2

Added config variable for External Network type in ML2

Description:
With the ML2 Plugin, every network created has segments with
provider:network_types being tenant_network_types.
When applied to external networks, the types that could be in
tenant_network_types parameter (like vxlan or gre) are not appropriate.

Implementation:
Added new config variable 'external_network_type' in ml2_conf.ini
which contains the default network type for external networks
when no provider attributes are specified, by default it is None.

It also includes small code re-factoring/renaming of import statement.

DocImpact

Closes-Bug: #1328991

Co-Authored-By: Romil Gupta <romilg@hp.com>
Change-Id: Idbbe6bced73cfedbe0f8e7abba35f87589b1a004