Optimize the calculation of length and hashes

[MVrachev]

Fixes issue #: None

Description of the changes being introduced by the pull request:
After we had given the option to use or not hashes and length
for timestamp and snapshot roles, it's good to make sure we are
calculating them only when they are needed.

This optimization could be important for the bigger tuf adopters.

Please verify and check that the pull request fulfills the following
requirements:

• The code follows the Code Style Guidelines
• Tests have been added for the bug fix or new feature
• Docs have been added for the bug fix or new feature

[MVrachev]

This pr uses the latest changes in securesyslib which are not yet released and that's why it fails on all python environments expect sslib-master.

Those changes don't need additional unit tests because there are multiple tests in tuf/tests/test_repository_lib.py which are already testing the different situations when length and hashes are/aren't used for timestamp and snapshot roles.

[joshuagl]

We'll need to update the minimum required securesystemslib in setup.py to be whichever released version includes the util.get_file_details() split

[jku]

What happened in the appveyor builds? The tests seem to take a couple of minutes on each run but the CI build won't finish until an hour later (which I think is appveyors timeout): 5 hours for the whole thing...

[trishankatdatadog]

@MVrachev let's mark this as a draft PR until ready, please? Thanks!

[MVrachev]

I converted it to a draft pr until a new release of securesyslib is released which includes the dependent functionality.

[joshuagl]

Great change, thanks @MVrachev. I'll try and ensure a securesystemslib release is made in the next week or so and then we should be good to merge this.

[MVrachev]

I updated this pr with the latest changes introduced on the develop branch and the necessary functions from securesyslib version 0.16.0 are already available.

The pr is ready for reviews.

PS: Will address the comments from Joshua.

[MVrachev]

I rebased on the recent changes in the develop branch and the tests should no longer fail, but I don't know why tox -e py27 fails...
Can somebody from the maintainers restart Travis CI?

[jku]

The TimeoutError was added by me, it was merged only today (#1096 ) ...

[joshuagl]

Lovely code cleanup, thanks @MVrachev !