Dollar Variable expansion (`$VAR`) was inadvertently disabled for
windows variables, although %VARIABLES% already worked. This reduced the
portability of hooks in general.

Additionally, tests would fail on windows due to bad quoting of paths:
the path C:/a/b/c would fail when passed to the command
`cmd /q /c C:/a/b/c` because it would interpret /a /b and /c as 3
options. Using quotes makes the tests pass.

When REBAR_CONFIG was set it would not effect the top level app's
configuration because app_discover was rereading the top level
rebar.config which ignored REBAR_CONFIG. Instead this patch has
it use the existing configuration from REBAR_CONFIG.

Test required `touch` utility to present on user's machine. Remove this dependency.

Test required `touch` utility to present on user's machine. Remove this dependency.