avocado.test: Support params passing to DropinTests

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>
@ldoktor you should use these environment variables defined with `self.params` directly on parameter `env`, defined in `process.run. It restricts the changes to the process running the test, so you just pass the extra vars and the environment will be updated.