Use dynamic local_path to accommodate Ansible running on VM

[fullyint]

Here are the two changes Windows users face after #705 and this PR #725:

1. Windows users who want to avoid the .vault_pass executable problem will need to run Ansible commands from ~/trellis (Create /home/vagrant/trellis bindfs mount with proper permissions #705).
2. Windows users will need to recognize that they no longer need to do anything about local_path (see https://discourse.roots.io/t/7825/7). They don't need to redefine local_path nor do they need to move the Vagrantfile (Use dynamic local_path to accommodate Ansible running on VM #725).

I'm not aware of any reason to NOT run Ansible commands from ~/trellis, so we could potentially disable the default /vagrant sync directory. That way we would just have a single option that works (~/trellis), instead of two options, one of which doesn't work with .vault_pass.

---

Here is a verbose representation of what #705 and #725 did and didn't change for Windows users:

	Before	After #705 (/vagrant)	After #705 (~/trellis)	After #725 (this PR)
Ansible succeeds with .vault_pass	no	no	yes	yes (if running in ~/trellis)
executable hosts are NOT a problem (fixed in Ansible 2.0 ansible/ansible#11643)	yes	yes	yes	yes
default local_path: ../site works without moving Vagrantfile	no	no	no	yes (dynamically changed)
users can redefine local_path: /srv/www/example.com/current	yes	yes	yes	yes (but unnecessary to redefine)
users can move Vagrantfile and run commands from /vagrant/trellis	yes	yes	yes	yes (but not needed for local_path)

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Details on permissions of vagrant sync directories.

                    ------ Permissions -----
                    Before #705   After #705
                    -----------   ----------
# Setup A (no winnfsd)
vagrant/
  .vaultpass         -rwxrwxrwx   -rwxrwxrwx   <-- 1a .vault_pass executable problem unchanged
  hosts/             drwxrwxrwx   drwxrwxrwx
    production       -rw-r--r--   -rwxrwxrwx   <-- 2a Ansible 2.0 allows hosts to be executable
                                                      https://github.com/ansible/ansible/pull/11643
                                                      so the extra sync dirs from #460 were removed
# Setup B (with winnfsd)
vagrant/
  .vaultpass         -rwxrwxrwx   -rwxrwxrwx   <-- 1b .vault_pass executable problem unchanged
  hosts/             drwxrwxrwx   drwxrwxrwx
    production       -rwxrwxrwx   -rwxrwxrwx   <-- 2b hosts executable, but not a problem

# Setup C (any)
~/trellis/
  .vault_pass                     -rw-r--r--   <-- 1c .vault_pass executable problem fixed
  hosts/                          drwxr-xr-x
    production                    -rw-r--r--   <-- 2c hosts changed to non-executable
  bin/                            drwxr-xr-x
    deploy.sh                     -rwxr-xr-x   <-- 3  scripts appropriately executable

The final Setup C is the recommended setup for Windows users. Run all your Ansible commands on the VM in ~/trellis, whether using winnfsd or not.

[swalkinshaw]

👍