Enable to specify nise_bosh revision

Add environment variable NISE_BOSH_REV to use specific revision of nise_bosh.
If it is not set, the revision currently checked-out is used.
Merge pull request #49 from nota-ja/feature-enable-to-specify-nise_bosh-revision

Enable to specify nise_bosh revision
Use `nats.machines`
Merge branch 'testing'
Merge branch 'master' of https://github.com/yudai/cf_nise_installer