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>
Merge pull request #288 from ldoktor/envtest

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