remove 'make publish' target (just run ./publish.sh instead)
build docs for every branch

For convenience, this patch also adds separate make targets for building
just the docs, the web site documentation, and the expensive release
artefacts.
add --skip-yum option for publish.sh, to skip building yum repos

This makes the publishing process much quicker, if you know that there
will be no changes to the yum repos (that is, if there hasn't been a new
Beaker release).

Change-Id: Iaa4e16cd5a4b741db51b3f2b4842174d9e32090e