Enable AUTOMAKE and ACLOCAL env variables for bootstrap.

The bootstrap script now supports an environment variable that points to
the AUTOMAKE, as well as to ACLOCAL, that you want to use to build Mesos.
It also validates that if you specify an env entry for AUTOMAKE you do
the same for ACLOCAL.

[ticket: MESOS-849]: As a developer I should be able to set the AUTOMAKE
and ACLOCAL environment variables for autoconf to pickup when using the
bootstrap script.
Merge branch 'feature/MESOS-849' of github.com:berngp/mesos into feature/MESOS-849