ipv6 support

[OmAsana]

The tool does not work with ipv6 only hosts.
In the log I see the following line:

[2017-05-22 15:38:29,314] [CRITICAL] [MainProcess] [Main:exception:218] Error setting up mongodb-consistent-backup: Could not resolve host 'myhostname', error: [Errno -2] Name or service not known

The problem is that the function validate_hostname() here does not support ipv6.

Mongodump tool supports ipv6 out of the box. For testing purposes I simply commented out this check and backups were done without problems.