Edge driver: Improve exception handling

Improve exception handling in the edge driver code.

Partial-Bug: #1305032

Change-Id: I727a7389aff216c4347de5605b8b1979d3e5a94e
Verify ML2 type driver exists before calling del

Verifies that an ML2 type driver exists for a given
segment type before attempting to call the release_segment
method on it. Logs an error if a type driver is not
found.

This covers the case where a segment is created with
a given type and then ML2 is reconfigured without
support for that type.

DocImpact
The ML2 documentation should be updated with a warning
that disabling a network type driver and re-enabling
it later may lead to possible DB inconsistencies.

Closes-Bug: #1292102
Change-Id: I65a35abf3ed57347bb6e8fee228f8c0697217c00
Merge "Edge driver: Improve exception handling"
Merge "Verify ML2 type driver exists before calling del"