OFAgent: Process port_update notifications in the main agent loop

Port the following patch to OFAgent.
commit: 5e6e592132aa9a98936ce3bfdb66efc7832caafb
https://review.openstack.org/#/c/61964/

Partial-Bug: 1293265

Change-Id: I53813d12c66dc746cd373fd91ff9bd9bdbf222db
OFAgent: Improve handling of security group updates

Port the following patch to OFAgent.
commit: 3046c4ae22b10f9e4fa83a47bfe089554d4a4681
https://review.openstack.org/#/c/63100/

Partial-Bug: 1293265

Change-Id: Iecdfee894ff2fd5f05f63f040dd70821af124737
Make help texts more descriptive in Metaplugin

This patch makes help texts of configuration parameters of
metaplugin.ini more descriptive. Comments in metaplugin.ini
are also improved.

Change-Id: I1fac888ecc223a7199cc9c9aeb603b7493f22171
Closes-Bug: #1283930
Merge "Make help texts more descriptive in Metaplugin"
Merge "OFAgent: Process port_update notifications in the main agent loop"
Merge "OFAgent: Improve handling of security group updates"