retry getting blob in go agent

Signed-off-by: Maria Shaldibina <mariash@pivotallabs.com>
renamed sha1Verifiable to sha1VerifiableBlobstore

Signed-off-by: Maria Shaldibina <mariash@pivotallabs.com>
suffix blobstores with blobstore

Signed-off-by: Maria Shaldibina <mariash@pivotallabs.com>