silo benchmark fails behind proxy
From @mateusz-blaszkowski in #475: 

> silo - failed with Clone of 'git://github.com/kohler/masstree-beta.git' into submodule path 'masstree' failed. I run the test behind the proxy and this is the case. I would have changed the path to Git repository to https:// but it is hidden somewhere in 'dbtest' (look a the command which failed: cd /tmp/pkb/silo && MODE=perf DEBUG=0 CHECK_INVARIANTS=0 make -j80 dbtest). Oh, i found that the exact path is specified here: https://github.com/stephentu/silo/blob/cc11ca1ea949ef266ee12a9b1c310392519d9e3b/.gitmodules

We should switch it to `https://`.

diff --git a/perfkitbenchmarker/linux_packages/silo.py b/perfkitbenchmarker/linux_packages/silo.py
index 2c14a0bc..fdb64fa6 100644
--- a/perfkitbenchmarker/linux_packages/silo.py
+++ b/perfkitbenchmarker/linux_packages/silo.py
@@ -33,6 +33,9 @@ def _Install(vm):
   vm.RemoteCommand('git clone {0} {1}'.format(GIT_REPO, SILO_DIR))
   vm.RemoteCommand('cd {0} && git checkout {1}'.format(SILO_DIR,
                                                        GIT_TAG))
+  # This is due to a failing clone command when executing behind a proxy.
+  # Replacing the protocol to https instead of git fixes the issue.
+  vm.RemoteCommand('git config --global url."https://".insteadOf git://')
   vm.RemoteCommand('cd {0} && MODE=perf DEBUG=0 CHECK_INVARIANTS=0 make\\
           -j{1} dbtest'.format(SILO_DIR, nthreads))
 
['perfkitbenchmarker/linux_packages/silo.py']