replace POLL_DELAY with actual configuration

[johrstrom]

The environment variable POLL_DELAY is used as a sort of secret option to specify how long to wait before querying for jobs in the interactive sessions page. Let's replace this environment variable with an actual configuration folks can set.

Note that some sites may be using the environment variable - so we have to take care not to break that for them and just use precedence.

https://github.com/search?q=repo%3AOSC%2Fondemand%20POLL_DELAY&type=code