Support IPv6 in address check lookups.

This supports IPv6 in address DNS lookups and socket bind checks. Addresses are
looked up with the system getaddrinfo(), and the v6/v4 preference of this lookup
is respected. IPv6 address literals in the config file are expected to be
bracketed in [], and the result of lookups will be bracketed as well, to support
the ZeroMQ URI format.
Unset the IPV4ONLY socket option if available.

Since IPv6 sockets support both v4 and v6 addresses, the socket option is unset
unconditionally if the parameter is available in the zmq module. The actual
decision as to which address version to use is done by the address lookup code,
which obeys the system v4/v6 preference.
Be more resilient towards missing addresses and hostname lookup failures.
Add a unit test for passing IPv6-formatted addresses to verify_socket.
IPv6 address literals should be specified without brackets in config files.
delete `salt`. the sql statements, because "conn" is  already specified
in "_get_serv" function. If specify it again, the mysql.db in
config.option is not efficacious
Whoops :) return False, not None!
Update ubuntu.rst

The `software-properties-common` package is required to be able to do `add-apt-repository` and not the mentioned `python-software-properties`.
Merge pull request #4545 from syphernl/patch-1

Update ubuntu.rst
Merge pull request #4506 from pengyao/develop

delete `salt`. the sql statements
Add user wildcard matching for external auth
Include the filesystem cleanup options group into the parser.
Merge pull request #4547 from sfdc-kho/develop

Add user wildcard matching for external auth
Merge pull request #4548 from s0undt3ch/develop

Include the filesystem cleanup options group into the parser.
Fix for #4514

Cleaning up warning/error messages for issue #4514.  There are now
log messages when either aptsources.sourceslist or
softwareproperties.ppa fail to import.  There are also clearer
log warnings when trying to use functions from either.
Allow for the main session key to be updated hot
Add dropfile function to utils
Move to salt.crypt
Fix #4520

Call for a refresh on the publish session key when removing keys
and rejecting keys forcing rejected minions to re-auth
Pass in the cachedir to dropfile