Default configuration tox env name change

Currently tox configuration has env by name "custom", renaming it
to "default" to generalize for all supported system.

Going forward there will be no system model concept supported from
the test env or from developement skeleton model.

Resolves openbmc/openbmc-test-automation#645

Change-Id: I0b678ffc10e373214bd1dec425182462955acb2c
Signed-off-by: George Keishing <gkeishin@in.ibm.com>
2 files changed