FEAT: new custom filter 'fstab_mounted.dirs' to ls_lan spec

[TZ3070]

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Complete Description of Additions/Changes:

• add a special filter keyword fstab_mounted.dirs to the ls_lan spec it's not a real directory existing in any system, but a switch like keyword, to enable the ls_lan_dirs to involve the mounted points in the default /etc/fstab configuration file as the ls -lan candidates.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 77.14%. Comparing base (5c68edd) to head (03d352f). Report is 2 commits behind head on master.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #4255 +/- ## ======================================= Coverage 77.14% 77.14% ======================================= Files 762 762 Lines 41476 41483 +7 Branches 8763 8765 +2 ======================================= + Hits 31997 32004 +7 + Misses 8422 8421 -1 - Partials 1057 1058 +1 ``` | [Flag](https://app.codecov.io/gh/RedHatInsights/insights-core/pull/4255/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=RedHatInsights) | Coverage Δ | | |---|---|---| | [unittests](https://app.codecov.io/gh/RedHatInsights/insights-core/pull/4255/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=RedHatInsights) | `77.13% <100.00%> (+<0.01%)` | :arrow_up: | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=RedHatInsights#carryforward-flags-in-the-pull-request-comment) to find out more.

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