CLI --sign: Support signing of delegated metadata and ability to import any public key type

[vladimir-v-diaz]

Fixes issue #:

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Description of the changes being introduced by the pull request:

This pull request allows any delegated role to be signed with the --sign option (it previously only supported signing of the Targets role). Public keys of any key type can now be imported, as well.

Unrelated: Allow 2+ roles to delegate to the same role. Although pending pull request #590 implements it, it is implemented here to get everything working.

(env) $ repo.py --init
(env) $ repo.py --key ed25519 --filename foo_key
(env) $ repo.py --delegate "foo*.tgz" --delegatee foo --pubkeys tufkeystore/foo_key.pub
(env) $ repo.py --sign tufkeystore/foo_key --role foo
Enter a password for the encrypted key (tufkeystore/foo_key):
(env) $ cat tufrepo/metadata/foo.json
{
 "signatures": [
  {
   "keyid": "93d511347a13c2c3bec4537bbe5b642c7860c4f2590dfec6470182628404699f",
   "sig": "7f78b552560b589814cd7a7fc49047a855660d576c22fc9e6b83b55cf733d0516a07bd664993ef48908e7bff6770395940cc07b5f3529cbc97f2253face07b0f"
  }
 ],
 "signed": {
  "_type": "targets",
  "delegations": {
   "keys": {},
   "roles": []
  },
  "expires": "2018-06-05T05:53:53Z",
  "spec_version": "1.0",
  "targets": {},
  "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>