Nicira plugin: List ports on network gateways

Bug 1183277

This patch adds a read-only attribute to the network gateway API
extension. This attribute will return the list of ports connected to
a gateway, including their segmentation types and ids.
This patch now adds joined load for a gateway's connections, thus
avoiding extra queries to the db.

Change-Id: I2b22c94eed5bc02be16771660008d31c74a8628e
Perform a sync with oslo-incubator.

This oslo-incubator sync pulls in a new log.py which will make quantum's
default log output format the same as nova, glance and cinder (once
cinder's corresponding oslo sync merges). This common log format
simplifies log indexing as part of CI and makes lives easier for
deployers.

This sync does add a requirement on six as jsonutils depends on it. It
updates install_venv_common.py to be python26 compatible. It also brings
in a bunch of recent python3 compatibility that was added to oslo.

Fixes bug 1183144

Change-Id: Id0f196d7b5680e5950e4a27d66042bf00ccd49e6
Added conversion specifier for proper log

In function create_for_sg_rule conversion specifier after string varaible
"rule_id" was missed. It was causing an error during translation of debug
message due to unavailability of conversion specifier. Included proper
conversion specifier for getting proper debug log.

Fixes: bug #1183734

Change-Id: I0342a091fb8199273c1bb9af59c42c0e09c39349
Merge "Perform a sync with oslo-incubator."
Merge "Nicira plugin: List ports on network gateways"
Merge "Added conversion specifier for proper log"