libcloud plugin: libcloud storage provider config

[pstorz]

Is a port of PR #1174 to master

Right now S3 is hardcoded within the provider type
for the apache libcloud plugin. Any other value, in
the config_file parameter for "provider" will not
be respected.

With this change the option will be actually used.

(cherry picked from commit ae36762)

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[bruno-at-bareos]

Nice work, thanks contributing.