install-autotest-server.sh: Allow more install possibilities

The install server script so far was rather unflexible:

 * Only the reference repo, master branch, could be installed
 * Only /usr/local/autotest could be used as install root

So rework the script a bit so that it is possible to:

 * Use an arbitrary autotest repo
 * Use an arbitrary autotest branch
 * Use an arbitrary autotest base dir

With this, it will be possible to automate server deployment
testing, a big win in the regular next testing of our code.

Signed-off-by: Lucas Meneghel Rodrigues <lmr@redhat.com>
install-autotest-server.sh: Introduce error contexts

Introduce error contexts, which are ways to tell what
was the context when a given operation failed. Create
a log_context function, then change it to log the main
parts of the server install. Later on we'll introduce
a function to run commands, log their outputs and
fail the script in case they go wrong, that will
benefit from it.

Signed-off-by: Lucas Meneghel Rodrigues <lmr@redhat.com>