Config for parallel backup

Problem description
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Currently it is impossible to specify multiple storages for parallel backup.
Currently all parameters for swift storage are taken from environment variables, so for multiple swift storages we should provide credentials through file.

Solution
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Proposed change allows to specify multiple "[storage]" sections inside of config file. So for using freezer with multiple backup storages user should use config file (cli interface still allows user to specify one storage).

Architecture
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From architecture point of view we have next changes:

1) Parsing additional sections in config file
2) Creation a storage per section. For swift storage - feature for extraction credentials from .osrc file.
3) Creation multiple storage, that incapsulates inside all intercommunication with multiple storages created on step 2

This way allows to add support of multiple storages without a major code refactoring. All components should be agnostic about type of storage backend (multiple or single).