autotest: don't source the configuration file in every test script

Most of our bash test scripts source autotest_common.sh
to be able to use some autotest-specific functions like
timing_enter(). The same test scripts allow specifying
custom command line parameters without actually realizing
that those parameters can be potentially picked up by
autotest_common.sh as well.

For example, if particular nvmf tests are run in "isolation"
mode by being executed with the first param set to "iso",
and there is a file named "iso" in the current dir, that
file will be sourced. This could be bad.

In this patch we stop sourcing or even processing $1 in
autotest_common.sh. Instead, the test configuration will
be sourced just once from autobuild.sh, autopackage.sh
and autotest.sh. If the user wants to run particular test
scripts manually, he should source an SPDK test configuration
by himself - manually as well. In most cases he won't even
have two, as only a few test scripts depend on SPDK_*
variables.

Note that we still have to setup the default values for
SPDK_* variables in autotest_common.sh because some of
our test scripts actually depend on them:

> if [ $SPDK_TEST_RBD -eq 1 ]; then ...

Because it lacks any type of quotes around SPDK_TEST_RBD,
it will print the following message when that variable
is unset:

> /bin/bash: line 0: [: -eq: unary operator expected

It doesn't trigger any error ($? == 0), but can be still
a bit misleading in the script output.

Change-Id: I350045d8582d66fe1ed7697d4bcbba324cb541ad
Signed-off-by: Darek Stojaczyk <dariusz.stojaczyk@intel.com>
Reviewed-on: https://review.gerrithub.io/c/spdk/spdk/+/453876


Tested-by: SPDK CI Jenkins <sys_sgci@intel.com>
Reviewed-by: Ben Walker <benjamin.walker@intel.com>
Reviewed-by: Jim Harris <james.r.harris@intel.com>