Revert "Revert "remove map and unmap domain""

This reverts commit f62eb6eb7bd3afb23c49e307777be10f7c9aa82e.

Signed-off-by: Stephen Levine <slevine@pivotallabs.com>

Conflicts:
	src/cf/commands/domain/domain_mapper_test.go
	src/testhelpers/api/fake_domain_repo.go