Consider available disk before deciding where to start app.

CC currently assumes that if a DEA has enough memory, it has enough disk
space. This doesn't work in all cases, especially with larger buildpacks
or apps with larger disk_quota settings. This change considers the
available disk space on a DEA before making the placement decision.
Merge pull request #120 from julz/disk_space_dea_locator

Consider available disk before deciding where to start app.