It's quite useful to be able to configure containers by specifying
environment variables, which init (or initscripts) can use to adjust the
container's operation.

This patch adds one new configuration parameter, `lxc.environment`, which
can be specified zero or more times to define env vars to set in the
container, like this:

    lxc.environment = APP_ENV=production
    lxc.environment = SYSLOG_SERVER=192.0.2.42
    lxc.environment = SOMETHING_FUNNY=platypus

Default operation is unchanged; if the user doesn't specify any
lxc.environment parameters, the container environment will be what it is
today ('container=lxc').
Signed-off-by: Matt Palmer <mpalmer@hezmatt.org>
Acked-by: Stéphane Graber <stgraber@ubuntu.com>
Acked-by: Serge E. Hallyn <serge.hallyn@ubuntu.com>