bcrypt hash GPG passphrase for key stretching

Implements https://github.com/freedomofpress/securedrop/issues/171.
Minor change to setup_ubuntu.sh to it make it work when there are
multiple keys in the journalist keyring (happens sometimes
during testing).
Add runtime test BCRYPT_GPG_SALT != BCRYPT_ID_SALT

If BCRYPT_GPG_SALT and BCRYPT_ID_SALT are the same, the source
GPG passphrase is no longer secure. We should add tests to
check for unlikely conditions like this one that might arise
at runtime.
Merge branch 'master' into feature/bcrypt-gpg-passphrase

Conflicts:
	install_files/document.config.py
	install_files/interface_install.sh
Do runtime tests on crypto_util import
Merge pull request #178 from diracdeltas/feature/bcrypt-gpg-passphrase

bcrypt hash gpg passphrases closes #171