tox: override CCACHE_DIR for due to fishfish hack
tox: override CCACHE_DIR for due to fishfish hack

due to HOME=fishfish hack (for tests which require touching $HOME)
we need to override also CCACHE_DIR, as otherwise ccache
during pip installations fails with fishfish/.ccache not existing.

Change-Id: Ie089cdd073f2f342bc8845620ca9fcbda2a8fc34

redhat-openstack/infrared,okolisny/InfraRed,redhat-openstack/infrared,okolisny/InfraRed,redhat-openstack/infrared,okolisny/InfraRed