Added support for STS security tokens.

[kvij]

I have added an optional parameter aws_security_token to the Route53Connection class so EC2 profiles with sufficient permissions can use the library without storing credentials.

I use the following snippet to resolve credentials but I'm not sure this should be part of an API library.

import os
import requests

__author__ = "Karel van IJperen"

class Credentials(object):
    """Amazon AWS Credentials resolver. First check parameters.
    Than environment. And than try to get the instance credentials
    following properties:

    Attributes:
        access_key_id: A string representing the id.
        secret_access_key: A string representing the HMAC secret.
        token: A string representing the STS token.
        expiration: A string representing the expiration date that comes with temporary security credentials.
    """
    def __init__(self, access_key_id = None, secret_access_key = None, profile = None):
        if access_key_id and secret_access_key:
            self._set_attributes(access_key_id,
                                 secret_access_key)
            return

        if profile:
            self._credentials_from_profile(profile)
            return

        if (os.environ.get('AWS_ACCESS_KEY_ID', False) and
            os.environ.get('AWS_SECRET_ACCESS_KEY', False)):
            self._set_attributes(os.environ.get('AWS_ACCESS_KEY_ID'),
                                  os.environ.get('AWS_SECRET_ACCESS_KEY'))
            return

        profile = self._get_profile()
        self._credentials_from_profile(profile)

    def _get_profile(self):
        r = requests.get('http://169.254.169.254/latest/meta-data/iam/security-credentials/')
        if r.status_code == 200:
            return r.text

    def _credentials_from_profile(self, profile):
        if profile:
            r = requests.get('http://169.254.169.254/latest/meta-data/iam/security-credentials/'
                             + profile)
            sts = r.json()
            self._set_attributes(sts['AccessKeyId'],
                                 sts['SecretAccessKey'],
                                 sts['Token'],
                                 sts['Expiration'])

    def _set_attributes(self, access_key_id = None, secret_access_key = None,
                        token = None, expiration = None):
        self.access_key_id = access_key_id
        self.secret_access_key = secret_access_key
        self.token = token
        self.expiration = expiration