Check for the existence of a key.

    CLI example::

        salt myminion boto_kms.key_exists 'alias/mykey'

def key_exists(key_id, region=None, key=None, keyid=None, profile=None):
    '''
    Check for the existence of a key.

    CLI example::

        salt myminion boto_kms.key_exists 'alias/mykey'
    '''
    conn = _get_conn(region=region, key=key, keyid=keyid, profile=profile)

    r = {}
    try:
        key = conn.describe_key(key_id)
        # TODO: add to context cache
        r['result'] = True
    except boto.exception.BotoServerError as e:
        if isinstance(e, boto.kms.exceptions.NotFoundException):
            r['result'] = False
            return r
        r['error'] = __utils__['boto.get_error'](e)
    return r