Unify means of specifying environment to --env=<environment>

*If omitted & param is optional, defaults to 'dev'
*Supports arbitrary environment --env=foobar
*Deprecates support for `fhc restart appId live` or `fhc restart appId --live` - still supported but prints a warning

There'll be more work for FHC when things with dev/live specific endpoints (hosts `developmentHost`, env's `devValue`) are updated - deemed not in scope here. 
Will consider for when `fhc environments` exists, sure!