Use xz as compression tool for snapshot

[OpenStack Infra]

https://review.openstack.org/157344
commit a397cfe51887a163334ab12dc227090ce53125e7
Author: Dmitry Shulyak <email address hidden>
Date: Thu Feb 19 14:04:03 2015 +0200

    Use xz as compression tool for snapshot

    Default compression level will: -3

    Played with it on logs with ~6Gb of data

    Snapshot compressed with:
    - gzip takes: 479m Mb
    - xz 3 level - 272 and takes 4 minutes
    - xz 9 level - 239 and takes ~ 30 min

    It is possible to use pixz utility that will run compressing
    on multiple cores, but i dont think it is wise to depend on it,
    because when master host will be installed on vm - it may take
    very long time to generate snapshot

    DocImpact
    Change-Id: I84e66e92ebc20ceba2db98ff41f08b1fde323c0b
    Closes-Bug: 1394487

[Alexander Bozhenko (alexbozhenko)]

This feature will be introduced in 6.1, why it is moved to 7.0?

[Tomasz 'Zen' Napierala (tzn)]

It's a documentation bug, so it was decided it will be added in 7.0 documentation.