tests: set HOME env var

Travis CI's docs say that HOME is supposed to be set to /home/travis by
default, however, it also says "do not depend on this value":
http://docs.travis-ci.com/user/ci-environment/

Teuthology depends on the $HOME environment variable being set, and the
test suites that depended on this recently failed to build in Travis CI.

Set the var explicitly so it's always available for the tests.

.travis.yml

@@ -1,3 +1,5 @@
+env: HOME=/home/travis
+
 language: python
 python:
     - 2.7

tox.ini

@@ -2,6 +2,7 @@
 envlist = docs, py27, py27-integration, flake8
 
 [testenv:py27]
+passenv = HOME
 sitepackages=True
 deps=
   -r{toxinidir}/requirements.txt
@@ -14,6 +15,7 @@ deps=
 commands=py.test --cov=teuthology --cov-report=term -v {posargs:teuthology scripts}
 
 [testenv:py27-integration]
+passenv = HOME
 sitepackages=True
 deps=
   -r{toxinidir}/requirements.txt