This patch is initial support for people using custom bash scripts
with avocado. Tests should use:

    PATH=$(avocado "exec-path"):$PATH

and then they can utilize the helpers.

This version contain functions to write to Test.log the same way it's
possible from python including failing the test with TestWarn in case
avocado_warn was used.
Signed-off-by: NLukáš Doktor <ldoktor@redhat.com>