fix: do not make the first pool active by default (#275)

Description

Do not make the first pool active by default, as this causes the empty pool to be mistakenly marked as active:

...
2021/10/13 03:08:57 pool_manager.go:218: [INFO]   Available storage pools:  [first second]
2021/10/13 03:08:57 pool_manager.go:219: [INFO]   Active pool:  first
...
2021/10/13 03:09:00 retrieval.go:108: [INFO]   Pool to perform a full refresh:  second
...

However, at the moment both are empty

Related issue

#275 (closed)

Examples

After the fix:

...
2021/10/13 03:11:29 pool_manager.go:218: [INFO]   Available storage pools:  [first second]
2021/10/13 03:11:29 pool_manager.go:219: [INFO]   Active pool:  first
...
2021/10/13 03:11:29 retrieval.go:108: [INFO]   Pool to perform a full refresh:  first
...

Checklist

• the MR description has been reviewed
• this MR contains text changes and they have been reviewed OR there are no texts changes
• this MR contains GUI/CLI changes and they have been reviewed OR there are no GUI/CLI changes
• this MR contains API changes, specifications reflect those changes and they have been reviewed OR there are no API changes

Closes #275 (closed)