Consolidate all execs in fuelmenu

[Matthew Mosesohn]

All subprocess execs of shell commands should be called under a single function, instead of using a variety of hacked up methods.

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.openstack.org/272107

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.openstack.org/272107
Committed: https://git.openstack.org/cgit/openstack/fuel-menu/commit/?id=d2a14d489850da1327ab9aa5281c7f37d6d00db9
Submitter: Jenkins
Branch: master

commit d2a14d489850da1327ab9aa5281c7f37d6d00db9
Author: Matthew Mosesohn <email address hidden>
Date: Mon Jan 25 17:16:57 2016 +0300

    Consolidate externall shell execs

    Individual subprocess Popen and communicate calls are
    now directed through utils.execute() method.

    Fixed gateway deletion shell command in case there is
    a metric in the command output.

    Change-Id: Ic9f3432d8dc511dea0fe892a0828d70f5cf7eca7
    Closes-Bug: #1514387

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.openstack.org/275271

[Matthew Mosesohn (raytrac3r)]

This bug is still fix committed. 275271 is related to bug #1538757