avocado.test: Support params passing to DropinTests [v2]

This patch injects all params as env variables to
DropinTests. This is very useful for multiplexing
these tests.

Additionally the env_variables.sh example test was
adjusted to print CUSTOM_VARIABLE, which can be used
in selftest.

Signed-off-by: Lukáš Doktor <ldoktor@redhat.com>

v1: https://github.com/avocado-framework/avocado/pull/276

Changes:

    v2: pass new env as process.run() parameter
    v2: updated documentation