Parse metadata_service_num_attempts & metadata_service_timeout as integers. Fixes #395

This pull request fixes issue #395 by parsing integer values with `RawConfigParser.getint()`.

An alternative to this PR is to add `int()` to https://github.com/boto/botocore/blob/d4094511f7c5f084133578bb4d5a30befa7ccbe8/botocore/credentials.py#L47 or https://github.com/boto/botocore/blob/42b49b103662048fed22fc9debfbde78e11d5686/botocore/utils.py#L135, but this would fix the problem in a single place, rather than fixing the root cause, that is parsing.