[Nailgun] NetworkTemplate behaves incorrectly

[Ihor Kalnytskyi]

Current implementation of `NetworkTemplate` class is based on built-in `string.Template`. Unfortunately, the `string.Template` fits poorly our needs, since it's focuses on delimeter-based approach first when we need a braced one. Moreover, the `string.Template` has bugs in its implementation prior Python 2.7.10, and our `NetworkTemplate` behaves wrongly. Just check out this test -

   https://github.com/stackforge/fuel-web/blob/5aed5a7e9683e8b259c4437db413606125ec2ecd/nailgun/nailgun/test/unit/test_network_template.py#L53-L57

(we lost closes brace)

In order to make this class work properly on all Python interpreters, and reduce number of potential bugs (due to delimeter-based approach) let's reimplement it.

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.openstack.org/229011

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.openstack.org/229011
Committed: https://git.openstack.org/cgit/stackforge/fuel-web/commit/?id=180a39eae65300144a8f0a41e335b0845570d88b
Submitter: Jenkins
Branch: master

commit 180a39eae65300144a8f0a41e335b0845570d88b
Author: Igor Kalnitsky <email address hidden>
Date: Tue Sep 29 18:15:05 2015 +0300

    Re-implement NetworkTemplate to use regex directly

    NetworkTemplate behaves wrong way in case we don't pass a substituation
    value or when closing pattern doesn't fit '%>' (check fixed tests). The
    main reason is that `string.Template` is designed to match delimeter
    based patterns, not braced ones. By some accident, the `string.Template`
    implementation is broken in both Python 2.6 and Python 2.7.6, and it's
    fixed for sure in Python 2.7.10 (because tests start to fail).

    Let's re-write our NetworkImplementation in order to have proper
    behaviour, and make tests to pass on various Py2 versions regardless of
    minor versions.

    Closes-Bug: #1500869

    Change-Id: I3f6e1cb7fb50ab9d3a47256b3e7e718b4f01e153
    Signed-off-by: Igor Kalnitsky <email address hidden>

[Dmitry Belyaninov (dbelyaninov)]

Verified by unit tests.