Adding default-series to example environments

[Curtis Hovey-Canonical]

I see two classes of confusion working with series that could be
alleviated is the "juju --init"  included default-series for every
environment. Users are trying Trusty, or trying to stay with Precise,
and they don't know about default-series.

Some users were deploying local charms without specifying the
series.These users didn't know that they could add default-series to
environments.yaml or do
    juju set-env default-series=precise
to a running env. [1]

Some users want run trusty environments from their precise clients.
default-series is the only way to specify the series of the bootstrap
node. Users have learned this through experimentation.

My own environment.yaml has default-series for some environments
because "juju init" has sometimes included it. Juju-CI always
specifies default series because we like to be clear about what we are
testing. I think every example environment for every provider needs a
commented out line explaining default-series
    # Set series for the state-server and the default series of charms.
    # default-series: trusty

[1] The juju docs are updated. We are waiting for
https://juju.ubuntu.com/docs to get the next update.

-- 
Curtis Hovey
Canonical Cloud Development and Operations
http://launchpad.net/~sinzui