metadatasvc: changed sha1 to sha256

Although HMAC with sha1 is less affected by collisions than sha1 alone I personally think it's better to use sha256 since the beginning.
This has also the advantage of automatically increasing the key sizei, for the HMAC function, from 160 bits to 256, which consequently increases the HMAC strength.
metadatasvc: wrapped initCrypto with log.Fatal

In case the HMAC key generation fails then do not allow the metadatasvc process execution to continue and print the error.
This avoids any potential security issues if there is not enough entropy in the system.
metadatasvc: checking the initCrypto error

Missed the real checking for the initCrypto error.
I have also taken the liberty to slightly change the printing of the error of the setupIPTables call to keep it consistant and use log.Fatal
metadatasvc: changed from fmt to log the setupIPTables

To keep the error handling consistant on the main function I changed the printing of the error from fmt + return to log.Fatal in the setupIPTables
*: remove historical RAF references
Cleaned up if statement
Merge pull request #135 from silenteh/metadatasvc-sha256

metadatasvc: changed sha1 to sha256
Merge pull request #138 from silenteh/metadatasvc-initcrypto

metadatasvc: wrapped initCrypto with log.Fatal
Merge pull request #163 from levigross/master

stage1: simplify if statement
Merge pull request #150 from jonboulle/raf

Remove deprecated "RAF" term
Merge pull request #154 from jonboulle/master

*: use TeeReader to reduce I/O in CAS
appc/spec: clarify AC_APP_NAME
spec: fix typo in app manifest.json
Merge pull request #171 from deepak1556/spec_patch

spec: fix typo in app manifest.json
stage1: gpg verify and cache retrieved coreos image
*: use types.Hash to refer to images
Merge pull request #176 from jonboulle/spec

appc/spec: clarify AC_APP_NAME
Merge pull request #175 from jonboulle/hash

*: use types.Hash to refer to images
Merge pull request #174 from vcaputo/cache_pxe_img

stage1: gpg verify and cache retrieved coreos image
appc/spec: make version a first-class required field