Updates repository_lib to use new securesystemslib interface (fix #412)

[lukpueh]

Fixes #412

• securesystemslib copies functions to generate and import keys from repository_lib.
  This PR replaces those function implementations with calls to securesystemslib.interface.

• Removes internal _prompt and _get_password from repository_lib because they were only used by the removed function bodies and also exist in securesystemslib.interface.

• Updates requirements.txt and setup.py to point to a securesystemslib branch that updates key generate and import functions to return a key object with the optional keyid_hash_algorithms field.

ONLY MERGE AFTER lukpueh/securesystemslib/update-import-keys has merged into securesystemslib/master and above references have been updated!!

(@SantiagoTorres secretly dances the we-should-have-used-git-submodules-dance)

[lukpueh]

And here is the shell snippet:

>>> import tuf.repository_lib as rl
>>> rl.generate_and_write_ed25519_keypair('testkey', 'pw')
>>> rl.import_ed25519_privatekey_from_file('testkey', 'pw')
{
  u'keytype': u'ed25519',
  u'keyid': u'f9d7e20448545d1ecab27c2a3011ab6332e974054c83818c9550690c26a7e02a',
  u'keyval': {
    u'public': u'9ce9a03e4dcd3fee4baa97393783fce76fc583d2896abcb0e556208f1a894ab6',
    u'private': u'19de8e160bf6d5df56715485c49a6c9683464a9388215b82ed2a0c2daa02d362'
  },
  u'keyid_hash_algorithms': [u'sha256', u'sha512']
}
>>> rl.import_ed25519_publickey_from_file('testkey.pub')
{
  u'keytype': u'ed25519',
  u'keyid': 'f9d7e20448545d1ecab27c2a3011ab6332e974054c83818c9550690c26a7e02a',
  u'keyval': {
    u'public': u'9ce9a03e4dcd3fee4baa97393783fce76fc583d2896abcb0e556208f1a894ab6'
  },
  u'keyid_hash_algorithms': [u'sha256', u'sha512']
}

[coveralls]

Coverage decreased (-0.03%) to 97.919% when pulling 3982a56 on lukpueh:fix-#412 into 6fde622 on theupdateframework:develop.