Support providing env vars to container init

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>