Only allow english letter characters when validating a digest
Only allow english letter characters when validating a digest

[#136289377](https://www.pivotaltracker.com/story/show/136289377)

Signed-off-by: Dennis Leon <84f467a5e250ceeacb4292b28bf3ac3f822ea696@pivotal.io>

cloudfoundry/bosh-utils,cloudfoundry/bosh-utils