Rally bash completion does not allow to auto-complete file path

[Alexander Maretskiy]

Regular shell prompt allows to auto-complete files paths (the most used case is path to task file), however that does not work with rally bash completion

Steps to reproduce:

 * Having rally and some task file, for example foo.json
 * type "rally task start " and that press TAB TAB to get some auto-complete choices

  Expected result:
    having choices that include "foo.json"

  Actual result:
    the only choices are rally arguments:
    --abort-on-sla-failure --deployment --no-use --tag --task --task-args --task-args-file

 * type "rally task start --task" and again press TAB TAB to get completion choices

  Expected result:
    having choices that include "foo.json"

  Actual result:
    the only choices are rally arguments:
    --abort-on-sla-failure --deployment --no-use --tag --task --task-args --task-args-file

 * then "rally task start --task foo" and again press TAB TAB to get completion choices

  Expected result:
    having auto-completion to "rally task start --task foo.json"

  Actual result:
    nothing happens

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.openstack.org/197186
Committed: https://git.openstack.org/cgit/openstack/rally/commit/?id=ca34e1ea2d56bd960ffe902be69325cbd3c077c5
Submitter: Jenkins
Branch: master

commit ca34e1ea2d56bd960ffe902be69325cbd3c077c5
Author: Alexander Maretskiy <email address hidden>
Date: Tue Jun 30 19:44:11 2015 +0300

    [Bash-Completion] Fix bash completon for files paths

    Changes:
      * fix for command `rally task start --task [TAB]' - now
        it shows filepaths choices
      * introduces function _rally_filedir as replacement for
        _filedir - this fixes issue `._filedir: command not found'
        in case if /etc/bash_completion routines are not loaded
      * small refactoring in rally bash completion

    Change-Id: Ib5e15a20c52ba3ccec32424d9404d2a929d82958
    Closes-Bug: #1470006