salt.modules.gpg: allow getting keys by short key ID

[vutny]

What does this PR do?

It restores the ability of the gpg exec module to handle short GnuPG key IDs (last 8 chars of the fingerprint) in such functions as get_key, get_secret_key, delete_key and trust_key. Also, fixed some docstrings.

What issues does this PR fix or reference?

Modern versions of python-gnupg module always return long key ID (16 chars) as a result of calling list_keys() method. But the documentation for the Salt gpg module clearly says that you're able to use short key IDs in the example sections. This PR allows to use both short and long key ID as an argument for the functions mentioned above.

Previous Behavior

I've imported public key into my keyring.

# salt-call gpg.list_keys
local:
    |_
      ----------
      created:
          2011-10-13
      expires:
          2019-07-02
      fingerprint:
          B97B0AFCAA1A47F044F244A07FCC7D46ACCC4CF8
      keyLength:
          4096
      keyid:
          7FCC7D46ACCC4CF8
      ownerTrust:
          Unknown
      trust:
          Unknown
      uids:
          - PostgreSQL Debian Repository

But, unfortunately, I can't get information about it directly.

# salt-call gpg.get_key ACCC4CF8
local:
    False

New Behavior

Now gpg module supports short key IDs as well:

# salt-call gpg.get_key ACCC4CF8
local:
    ----------
    created:
        2011-10-13
    expires:
        2019-07-02
    fingerprint:
        B97B0AFCAA1A47F044F244A07FCC7D46ACCC4CF8
    keyLength:
        4096
    keyid:
        7FCC7D46ACCC4CF8
    ownerTrust:
        Unknown
    trust:
        Unknown
    uids:
        - PostgreSQL Debian Repository

Tests written?

No

[cachedout]

Thank you, @vutny