When stretching the key for vault files, use PBKDF2HMAC() from the
cryptography package instead of pycrypto. This will speed up the opening
of vault files by ~10x.

The problem is here in lib/ansible/utils/vault.py:

    hash_function = SHA256

    # make two keys and one iv
    pbkdf2_prf = lambda p, s: HMAC.new(p, s, hash_function).digest()

    derivedkey = PBKDF2(password, salt, dkLen=(2 * keylength) + ivlength,
                        count=10000, prf=pbkdf2_prf)

`PBKDF2()` calls a Python callback function (`pbkdf2_pr()`) 10000 times.
If one has several vault files, this will cause excessive start times
with `ansible` or `ansible-playbook` (we experience ~15 second startup
times).

Testing the original implementation in 1.9.2 with a vault file:

In [2]: %timeit v.decrypt(encrypted_data)
1 loops, best of 3: 265 ms per loop

Having a recent OpenSSL version and using the vault.py changes in this commit:

In [2]: %timeit v.decrypt(encrypted_data)
10 loops, best of 3: 23.2 ms per loop