Use venv to build documentation

The Sphinx documentation now make use of the `program-output` plugin
which would invoke commands to generate inline documentation. Ex:

    program-output:: zuul --help
    program-output:: zuul enqueue --help

We want the resulting output to correspond to Zuul source code, not the
command which is currently installed on the host running the doc.  On my
setup sphinx would die out because it cant find the command 'zuul'.

The new tox environement 'doc' runs sphinx in a virtual env which will
have the proper zuul command.

The generated doc is not written under the /.tox directory but to
/doc/build/html for convenience.

Example usage:

    tox -edoc && open doc/build/html/index.html

Change-Id: Ib0170f94bb2c09eb60e555a32e101e2e0959b18e
Content-Type can now be set for email-ext publisher

The email-ext publisher was always sending email using the default
content type which is server wide.  This tiny patch let us overwrite the
content-type on a job basis.

The YAML possible values are 'html' or 'text' to force the content-type
to one of the two modes.

Change-Id: Id66bc3369332c389dfb6318113392694a4f98aad
make command parsing compatible with python 2.6

python 2.6 ConfigParser has problems interpolating the None type so setting defaults to
empty string instead.

Closes-Bug: #1259631
Change-Id: Iceec1d8c3b77fd64ac54e02653800ad422778bdf
Merge "Use venv to build documentation"
Merge "Content-Type can now be set for email-ext publisher"
Merge "make command parsing compatible with python 2.6"
Minor fixes in the trigger documentation

Change-Id: I8ec3cae2c2d6086bbcda7d9e78fdab2af4b53052