Add hash_merge and hash_merge_recursive filters with documentation

This is based on some code from (closed) PR #7872, but reworked based on
suggestions by @abadger and the other core team members.

Closes #7872 by @darkk (hash_merge/hash_replace filters)
Closes #11153 by @telbizov (merged_dicts lookup plugin)

Don't compile .py files under .tox/

Remove deprecated and unused VaultAES encryption code

fixed hostvars access in conjunction with --limit usage

Note that this test was broken in devel because it was really just
duplicating the AES256 test because setting v.cipher_name to 'AES'
no longer selected AES after it was de-write-whitelisted.

Now that we've removed the VaultAES encryption code, we embed static
output from an earlier version and test that we can decrypt it.

Now that VaultLib always decides to use AES256 to encrypt, we don't need
this broken code any more. We need to be able to decrypt this format for
a while longer, but encryption support can be safely dropped.

fixes #12062

Fixes #11997

Also sets the max_fail_percentage value to the percent type.

ansible-lockdown added, minor editing

added info/link for ansible-lockdown to mailing list section, minor editing
(can't help myself it seems) to the paragraph about subscribing from a non-google account

fixes #12104

Vault cleanups, pass #1

Synchronize fix error

Now we don't have to recreate VaultEditor objects for each file, and so
on. It also paves the way towards specifying separate input and output
files later.

It's unused and unnecessary; VaultLib can decide for itself what cipher
to use when encrypting. There's no need (and no provision) for the user
to override the cipher via options, so there's no need for code to see
if that has been done either.

Fix the error:
 "RuntimeError: dictionary changed size during iteration"

Fixes #11981
Fixes #11995
Fixes #12039
Fixes #12077

'tox -e py26' would fail for me because python -m compileall would crawl
under .tox/py27 and, unsurprisingly, get SyntaxErrors on files from the
Python 2.7 standard library using syntax features not supported by
Python 2.6.

also normalized -M option across all cli
fixes #12016

This bug was introduced in commit f8bf2ba1 on July 27.  Hasn't gone out
in a release yet.

Unicode and other fixes for vault