Move datastore config handling into downloader

[srinibas-zededa]

The changes to datastore configuration, it does not take affect.

1. configure a datastore with bad key/password
2. publish an image to the device from this datastore
3. the download of the object fails
4. update the datastore configuration with good key/password.
5. the download of the object still failsl

Obseverved result: once the datastore configuration is bad, the object download always fails.
Expected result: update the datastore configuration, the object download should suceed.

Solution:

1. Remove the datastore configuration handling from zedagent/baseosmgr
2. Add datastore configuration handling to downloader.
3. With every change to datastore configuration,
   start the download of the failed downloadable objects, attached
   to that datastore.

https://wiki.lfedge.org/display/EVE/Proposal+for+moving+datastore+configuration+handling+to+Downloader

[srinibas-zededa]

Fixed with changes in pull request.

#176