[Standalone] Changed default mode of execution from 'consume' to 'reuse'

[JosepSampe]

The main reason of this change is that the reuse mode does not need config at all in the backend section, while the consume mode requires more parameters, so it makes more sense to define exec_mode: consume when there are more parameters in the config, instead of having to explicit set exec_mode: reuse alone in the backend section of the config.

At the same time, the are some cli commands (lithops clean, lithops worker list, lithops job list , lithops image build) that don't work because of the current default consume mode. With this change, they will all work by default without "tricks" in the code.

Developer's Certificate of Origin 1.1

   By making a contribution to this project, I certify that:

   (a) The contribution was created in whole or in part by me and I
       have the right to submit it under the Apache License 2.0; or

   (b) The contribution is based upon previous work that, to the best
       of my knowledge, is covered under an appropriate open source
       license and I have the right under that license to submit that
       work with modifications, whether created in whole or in part
       by me, under the same open source license (unless I am
       permitted to submit under a different license), as indicated
       in the file; or

   (c) The contribution was provided directly to me by some other
       person who certified (a), (b) or (c) and I have not modified
       it.

   (d) I understand and agree that this project and the contribution
       are public and that a record of the contribution (including all
       personal information I submit with it, including my sign-off) is
       maintained indefinitely and may be redistributed consistent with
       this project or the open source license(s) involved.