Allow Ansible 2.5.7

[tomassedovic]

This relaxes the Ansible version checking to allow 2.5.7.

While it is recommended to stick with 2.6, some of the packages are not
avaliable for systems running the current Long Term Support version of
OpenStack (Queens / OSP 13).

https://bugzilla.redhat.com/show_bug.cgi?id=1626538
https://bugzilla.redhat.com/show_bug.cgi?id=1635293

[openshift-ci-robot]

[APPROVALNOTIFIER] This PR is NOT APPROVED

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[tomassedovic]

I have tested this against OpenStack Queens with the following Ansible dependencies:

# requirements.txt 
openstacksdk==0.11.3
python-openstackclient===3.14.2
python-heatclient===1.14.0
osc-lib===1.9.0
os-client-config===1.29.0
shade==1.27.1
ansible==2.5.7
(ansible-2.5.7) 

These should match the versions in OSP 13 in https://bugzilla.redhat.com/show_bug.cgi?id=1626538.

If this gets accepted, I'll backport it to 3.11 so that can be run on OpenStack LTS as well.

[tzumainn]

I've tested this against queens as well (using packstack), and things seem to work!

[tomassedovic]

/cherrypick release-3.11

[openshift-cherrypick-robot]

@tomassedovic: once the present PR merges, I will cherry-pick it on top of release-3.11 in a new PR and assign it to you.

In response to this:

/cherrypick release-3.11

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[openshift-bot]

@tomassedovic: PR needs rebase.

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[sdodson]

/lgtm

[sdodson]

/cherrypick releae-3.11

[celebdor]

/cherrypick release-3.11