don't bypass the web of trust (#378) (#379)

* don't bypass the web of trust (#378)

the choice whether to use a web of trust and on which trust level is up
to the user of PGP/GPG and must not be overriden by tools that are set
on top

users can decide to ignore this safety net by setting their gpg.conf
adequately, defining an alias for `gpg --trust-model=always` or passing
the env GPG to blackbox in this way but we should not override their
preferences hardcoded

* update README

add note about the web of trust

* fix broken test

assume that we have `--quick-generate-key` if we run gpg2 instead of
doing a dry run for that (which has side effects that break the test)

bin/_blackbox_common.sh

@@ -253,7 +253,7 @@ function encrypt_file() {
   encrypted="$2"
 
   echo "========== Encrypting: $unencrypted" >&2
-  $GPG --use-agent --yes --trust-model=always --encrypt -o "$encrypted"  $(awk '{ print "-r" $1 }' < "$BB_ADMINS") "$unencrypted" >&2
+  $GPG --use-agent --yes --encrypt -o "$encrypted"  $(awk '{ print "-r" $1 }' < "$BB_ADMINS") "$unencrypted" >&2
   echo '========== Encrypting: DONE' >&2
 }