[plan-build] Is now `hab`-default for dependency installation.
[plan-build] Is now `hab`-default for dependency installation.

This is a behavior change for `hab-plan-build.sh`, but should hopefully
better serve users in the long term and is very unlikely to affect
anyone unless they are building stage1 sets.

The updated "pkg_installer" detection logic is:

* If `$NO_INSTALL_DEPS` is set, then installer will be used
* If `$HAB_BIN` is set, the program provided will be used. This affords
  a developer the ability to test with older/newer version of `hab` or
  alternative installers
* Otherwise, the `hab` found on `$PATH` when the program starts will be
  used and set internally as the `$HAB_BIN` value

Signed-off-by: Fletcher Nichol <fnichol@nichol.ca>

Pull request: #556
Approved by: reset

habitat-sh/habitat,rsertelon/habitat,georgemarshall/habitat,georgemarshall/habitat,habitat-sh/habitat,georgemarshall/habitat,georgemarshall/habitat,habitat-sh/habitat,habitat-sh/habitat,nathenharvey/habitat,rsertelon/habitat,rsertelon/habitat,nathenharvey/habitat,georgemarshall/habitat,habitat-sh/habitat,rsertelon/habitat,rsertelon/habitat,habitat-sh/habitat,rsertelon/habitat,georgemarshall/habitat,habitat-sh/habitat,georgemarshall/habitat,nathenharvey/habitat,habitat-sh/habitat,georgemarshall/habitat,nathenharvey/habitat,rsertelon/habitat,nathenharvey/habitat,nathenharvey/habitat