Re-submit "ML2 plugin should not delete ports on subnet deletion"

This patch was previously merged:

Commit: 0d131ff0e9964cb6a65f64809270f9d597c2d5d1

And reverted by this commit da00ed76e6008bd06dada0f0441ae436dd759cdf
in order to check if there was a relation with the gate failures.

Change-Id: Iacb4de8d9aa6a6cbe32c4f41fcf2657f2d09e6e2
Merge "Re-submit "ML2 plugin should not delete ports on subnet deletion""