interfaces: import interfaces with absolut path

Now imported from 'pyipmi.interfaces.Ipmitool' and 'pyipmi.interfaces.Aardvark'

Signed-off-by: Heiko Thiery <heiko.thiery@kontron.com>
interfaces: ipmitool: change variable names

Signed-off-by: Heiko Thiery <heiko.thiery@kontron.com>
tests: interfaces: initial unit tests for ipmitool

Signed-off-by: Heiko Thiery <heiko.thiery@kontron.com>