Add support for --lb-type through BBL_LB_TYPE

[cdutra]

• Allows users to set concourse lb type to bbl up environments for concourse

What is this change about?

This allows users to change their load balancer type to concourse if using bbl-up task to deploy Concourse.

Please provide contextual information.

Needed for #165162223

Please check all that apply for this PR:

• [ ] introduces a new task
• [x] changes an existing task
• [ ] changes the Dockerfile
• [ ] introduces a breaking change (other users will need to make manual changes when this is released)

Did you update the README as appropriate for this change?

• [ ] YES
• [ ] N/A

How should this change be described in release notes?

bbl-up task allows support for different Load Balancer types though BBL_LB_TYPE.

What is the level of urgency for publishing this change?

• [ ] Urgent - unblocks current or future work
• [x] Slightly Less than Urgent

Tag your pair, your PM, and/or team!

It's helpful to tag a few other folks on your team or your team alias in case we need to follow up later.

[cfdreddbot]

:white_check_mark: Hey cdutra! The commit authors and yourself have already signed the CLA.

[cf-gitbot]

We have created an issue in Pivotal Tracker to manage this:

https://www.pivotaltracker.com/story/show/165165450

The labels on this github issue will be updated when the story is started.