Enable space developer to see and map private domain

Signed-off-by: Kai Zhang <kaizhang@cn.ibm.com>
Refactor who can add domain to org logic.

Signed-off-by: Kai Zhang <kaizhang@cn.ibm.com>
Check md5 in droplet & buildpack cache uploads

If the Content-MD5 header is provided and its value
does not match the md5 of the upload, return a 400

Signed-off-by: David Varvel <dvarvel@pivotallabs.com>