Remove TEMPEST_CONFIG_DIR in the api tox env
Remove TEMPEST_CONFIG_DIR in the api tox env

Setting TEMPEST_CONFIG_DIR in tox.ini is breaking the gate,
probably because of a tox bug [1].
Removing the line to unblock the gate. This implies that
for local execution of the tests TEMPEST_CONFIG_DIR
should be set manually.

[1] https://bitbucket.org/hpk42/tox/issues/285/tox-220-breaks-some-toxini-config-files

Change-Id: If3742b6eabe295a5aaeb4901c55909a70c451698
Closes-bug: #1515335

cloudbase/neutron,mahak/neutron,sebrandon1/neutron,noironetworks/neutron,klmitch/neutron,huntxu/neutron,wolverineav/neutron,wolverineav/neutron,dims/neutron,eayunstack/neutron,mahak/neutron,bigswitch/neutron,apporc/neutron,igor-toga/local-snat,huntxu/neutron,chitr/neutron,klmitch/neutron,cloudbase/neutron,igor-toga/local-snat,openstack/neutron,sebrandon1/neutron,openstack/neutron,bigswitch/neutron,eayunstack/neutron,MaximNevrov/neutron,apporc/neutron,openstack/neutron,chitr/neutron,mahak/neutron,noironetworks/neutron,dims/neutron,MaximNevrov/neutron