Update load_repository() function to load the delegations metadata
starting from 'targets' and traversing downwards the delegated
roles in order to load correctly the delegations hierarchy.

Signed-off-by: Teodora Sechkova <tsechkova@vmware.com>

Add a tests case checking if delegated Targets() objects
are loaded correctly.

Signed-off-by: Teodora Sechkova <tsechkova@vmware.com>

Remove unnecessary list keeping track of loaded file names and
rewrite outdated comments.

Signed-off-by: Teodora Sechkova <tsechkova@vmware.com>

Replace the list used for the delegations graph traversal with
a deque and use a set to store already loaded roles and avoid
loops in case of cycles in the graph.
Improve comments and readability.

Signed-off-by: Teodora Sechkova <tsechkova@vmware.com>

Use the top-level targets object to reference already loaded
delegated targets instead of storing them in an additional
dictionary in load_repository().

Signed-off-by: Teodora Sechkova <tsechkova@vmware.com>

Rename repository_lib.get_metadata_filenames() and
get_delegations_filenames() to better match their
functionality and tuf terminology.

Signed-off-by: Teodora Sechkova <tsechkova@vmware.com>

Add TOP_LEVEL_ROLES as a global variable in roledb.

Signed-off-by: Teodora Sechkova <tsechkova@vmware.com>