Allow non-NFS Vagrant shared folders.

Default to true to maintain original behavior
Create test files outside of synced directory

The tests...

    external.filesystem.test_dump_storage.TestDumpStorage.testReadableOrThrow
    external.filesystem.test_json_dump_storage_permissions.TestJsonDumpStoragePermissions

were failing because files in synced (non-NFS) directories in Virtualbox
cannot correctly have their permissions changed. For example:

[vagrant@crash-stats socorro]$ pwd
/home/vagrant/src/socorro
<#> creat first foo in /home/vagrant/src/socorro -- a dir shared by the host OS
[vagrant@crash-stats socorro]$ touch foo

<#> create second foo in a dir local to the guest OS
[vagrant@crash-stats socorro]$ touch /tmp/foo

<#> look at perms
[vagrant@crash-stats socorro]$ ls -lah /tmp/foo && ls -lah foo
-rw-r--r-- 1 vagrant vagrant 0 Jun  8 01:18 /tmp/foo
-rwxrwxr-x 1 vagrant vagrant 0 Jun  8 01:18 foo

<#> mount point options for the synced folder are :mount_options => ['dmode=775', 'fmode=775']

<#> attempt to change perms
[vagrant@crash-stats socorro]$ chmod 0 /tmp/foo && chmod 0 foo

<#> confirm changes
[vagrant@crash-stats socorro]$ ls -lah /tmp/foo && ls -lah foo
---------- 1 vagrant vagrant 0 Jun  8 01:19 /tmp/foo
-rwxrwxr-x 1 vagrant vagrant 0 Jun  8 01:18 foo

<#> synced foo doesn't change, so socorro.unittest.external.filesystem.test_dump_storage.TestDumpStorage.testReadableOrThrow fails