Set 'st2_version' and 'box_version' dynamically or use env vars

[arm4b]

The PR adds the optional build params: environment variables could be passed to the build to control the StackStorm version and box/image version:

• env ST2_VERSION - x.y.z format, like 2.7.1 (default: fetch latest st2 stable release version if not set)
• env BOX_VERSION - YYYYMMDD format, like 20180131 (default: use today's date if not set)

This way we avoid hardcoding the versions and use ENV vars when the pinning is needed (for example when creating a GitHub release aka git tag like v2.7.1-20180507).

Nice to have: ST2_VERSION and BOX_VERSION format validation.

[arm4b]

found this requirement of version pinning while working on https://github.com/StackStorm/packer-st2/issues/10