Adds keyid_hash_algorithms to returned key objects

[lukpueh]

Fixes #36 and, once integrated and TUF is updated to use the new version, theupdateframework/python-tuf#412.

The functions listed below all return key objects that can carry an optional keyid_hash_algorithms field according to their schema. With this PR we always add the keyid_hash_algorithm field to the returned object, in:

interface.import_ed25519_privatekey_from_file
interface.import_ecdsa_privatekey_from_file
keys.generate_ecdsa_key
keys.import_rsakey_from_public_pem
keys.import_rsakey_from_pem
keys.import_ecdsakey_from_private_pem
keys.import_ecdsakey_from_public_pem

e.g.:

>>> from securesystemslib import interface
>>> interface.generate_and_write_ed25519_keypair('testkey', 'pw')
>>> interface.import_ed25519_privatekey_from_file('testkey', 'pw')
{
  u'keytype': u'ed25519',
  u'keyid': u'8f68ef937c031aad4862aa0bac8b45b719c4f2ba8f186b289ba66991f82c8041',
  u'keyval': {
    u'public': u'78fa9cf9b787beb754806c15cf88950a5e3c30ff8cad57dba79c623fd8c5ae00',
    u'private': u'f8919b50f6ea8c4e2060e6177970b7e8cde97950ff284973fbe54485e78689c1'
  },
  u'keyid_hash_algorithms': [u'sha256', u'sha512']
}

>>> interface.import_ed25519_publickey_from_file('testkey.pub')
{
  u'keytype': u'ed25519',
  u'keyid': '8f68ef937c031aad4862aa0bac8b45b719c4f2ba8f186b289ba66991f82c8041',
  u'keyval': {
    u'public': u'78fa9cf9b787beb754806c15cf88950a5e3c30ff8cad57dba79c623fd8c5ae00'
  },
  u'keyid_hash_algorithms': [u'sha256', u'sha512']
}

The PR also changes PUBLIC_KEY_SCHEMA to accept the optional keyid_hash_algorithm (like ANYKEY_SCHEMA).

The reason for adding this field is to be able to associate a key with e.g. a signature created by that key, but listing a different keyid, because a different keyid hash algorithm was used. By trying all possible algorithms, the key can still be matched.

[coveralls]

Coverage remained the same at 100.0% when pulling ceefa7c on lukpueh:update-import-keys into b535fc2 on secure-systems-lab:master.

[vladimir-v-diaz]

Can you please include a code snippet of the changed behavior, similar to how @Awaad demonstrated the problem in the original issue?
theupdateframework/python-tuf#412

[vladimir-v-diaz]

NOTE: I don't see any major issues with this pull request, but I will make minor edits to your pull request for any minor issues. I will share my in-line comments on this pull request for documentation purposes.

This SCHEMA is more like KEY_SCHEMA, since ANYKEY_SCHEMA contains an attribute (keyid) that ANYKEY_SCHEMA doesn't. PUBLIC_KEY_SCHEMA matches KEY_SCHEMA, and will now differ with the keyid_hash_algorithms and keyval attributes.

[vladimir-v-diaz]

Wrong object_name. Should be PUBLIC_KEY_SCHEMA.

[vladimir-v-diaz]

This should be "so that equal RSA keys ..."