Refactored server disk management.

Moved the code to get available disks into the get_mounted_disks function.
Added docstrings.
Added disk restoration function.
Added extra parameters to Swift replica checks

Added 'exact' param, so that it can be tested if Swift has
exactly 'count' number of replicas, no more, no less.

Added 'check_nodes' to be able to specify that all the primarily nodes have a
copy of a file (to do so, set check_nodes=replica_count) and other uses.
Added Swift test case "one disk down, restore it"
Added format_disk function
Fixed disk restoration

After mounting, restore permissions and SELinux security context.
Added info log messages to server management
Added test case "disk restoration"
Refactoring, replaced ssh.run() with ssh()

Made a __call__ method from the run() method for better readability.
Swift reset now keeps original permissions.