Generate Ed25519 keys by default. Address issue#727

[vladimir-v-diaz]

Fixes issue #:

Addresses #727.

Description of the changes being introduced by the pull request:

This pull request ensures that repo.py generates Ed25519 keys, by default. It should make it easier for clients to test a repo in pure Python (once secure-systems-lab/securesystemslib#149 is merged here).

repo.py --init generates Ed25519 keys for all the top-level roles.
repo.py --key generates an Ed25519 key.

(env) $ repo.py --init
(env) $ echo 'test file' > testfile
(env) $ repo.py --add testfile
(env) $ cat tufrepo/metadata/root.json
{
 "signatures": [
  {
   "keyid": "2627cc54117a2ca90cd8fc913914b05a8cd05d28d2363fc10205cb1a24751111",
   "sig": "62dae8c54424553066e1f853423bdd9d6ddfa852b88447a592579407ff95c019337a2dd44f16c61f66887aa64994510e02d2cd25e321bd3ca18eb94c4ea43a0b"
  }
 ],
 "signed": {
  "_type": "root",
  "consistent_snapshot": false,
  "expires": "2019-05-19T01:08:52Z",
  "keys": {
   "242a349ae9f390e6a572dd37c5f17025fc34b5bb8d040d72efc7bdec1121973a": {
    "keyid_hash_algorithms": [
     "sha256",
     "sha512"
    ],
    "keytype": "ed25519",
    "keyval": {
     "public": "ad0326133a4cb5524ceea5ea09a45dcab9dd5e4ea0dc128ecfcfcdfeb1f49785"
    },
    "scheme": "ed25519"
   },
   "2627cc54117a2ca90cd8fc913914b05a8cd05d28d2363fc10205cb1a24751111": {
    "keyid_hash_algorithms": [
     "sha256",
     "sha512"
    ],
    "keytype": "ed25519",
    "keyval": {
     "public": "cdf972ed925d49070fb54727fd6bbb936b87ba9265aa7ac20de13d6aa741a57d"
    },
    "scheme": "ed25519"
   },
   "30b0261ac7a778fb95166a957fe93f5abcdf6c7ea8f4e8f7f06041551c2f80d2": {
    "keyid_hash_algorithms": [
     "sha256",
     "sha512"
    ],
    "keytype": "ed25519",
    "keyval": {
     "public": "72b9df5202552dfeb32d73937af7143619343e503c9214012d1b8a58d0be34b9"
    },
    "scheme": "ed25519"
   },
   "999d1d75d9dfd1aafd825079dccd6d38e6a7677c5b748c0a47a785d4a11eadbb": {
    "keyid_hash_algorithms": [
     "sha256",
     "sha512"
    ],
    "keytype": "ed25519",
    "keyval": {
     "public": "02e575dcba532dc863e94294f116b5584e3d820df6941f5da103f0f1dd0cd5f2"
    },
    "scheme": "ed25519"
   }
  },
  "roles": {
   "root": {
    "keyids": [
     "2627cc54117a2ca90cd8fc913914b05a8cd05d28d2363fc10205cb1a24751111"
    ],
    "threshold": 1
   },
   "snapshot": {
    "keyids": [
     "242a349ae9f390e6a572dd37c5f17025fc34b5bb8d040d72efc7bdec1121973a"
    ],
    "threshold": 1
   },
   "targets": {
    "keyids": [
     "999d1d75d9dfd1aafd825079dccd6d38e6a7677c5b748c0a47a785d4a11eadbb"
    ],
    "threshold": 1
   },
   "timestamp": {
    "keyids": [
     "30b0261ac7a778fb95166a957fe93f5abcdf6c7ea8f4e8f7f06041551c2f80d2"
    ],
    "threshold": 1
   }
  },
  "spec_version": "1.0",
  "version": 1
 }
}(env) $

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

Signed-off-by: Vladimir Diaz <vladimir.v.diaz@gmail.com>