SOLAR_DB from env in testr execution if defined

Tox passes env SOLAR_DB to testenv,

Change-Id: Ib267fcd3b6df1133ac592f41aa4a9a407de23e03

.testr.conf

@@ -1,5 +1,5 @@
 [DEFAULT]
-test_command=SOLAR_DB="sqlite://" \
+test_command=SOLAR_DB=${SOLAR_DB:=sqlite://} \
              py.test ./solar --subunit $LISTOPT $IDOPTION
 test_id_option=--subunit-load-list=$IDFILE
 test_list_option=--collectonly

tox.ini

@@ -7,6 +7,7 @@ envlist = pep8,py27
 usedevelop = True
 install_command = pip install -U {opts} {packages}
 setenv = VIRTUAL_ENV={envdir}
+passenv = SOLAR_DB
 deps = -r{toxinidir}/test-requirements.txt
 commands = ostestr