Allow documentation to create sequence diagrams

Sequence diagrams are very useful thing for developer
documentation to include and show (especially seeing that
most of openstack is one big sequence diagram) so it would
be great to include and enable the sequence diagram sphinx
plugin and associated seqdiag software.

It will likely also be required to include:

https://bitbucket.org/blockdiag/seqdiag

Where the docs are being built (so there might be a infra
related changed needed to accomdate this).

Change-Id: I85c163265ea0a10881b5d34f601eea6147d4469d
Require Jinja2>=2.6

The Designate project requires Jinja2 functionality available only in
version 2.6+.

Additionally, add a comment specifying the Jinja2 license (BSD 3 Clause)

Partial-Bug: 1375613
Change-Id: I4cf8a38106385990b278475d6efc43245461ba6e
Add manila to projects.txt

Add manila and python-manilaclient to the list of projects.

A requirements gate job has been proposed as well and the two projects
do not contain any extra or incompatible requirements.

Change-Id: I621bf742211c640e08ee349a2f5d4d52bc8062e0
Merge "Add manila to projects.txt"
Merge "Require Jinja2>=2.6"
Merge "Allow documentation to create sequence diagrams"