Add new variable `LaTeX-default-document-environment'.

* latex.el (LaTeX-default-document-environment): New buffer-local
variable.
(LaTeX-default-environment): Mention
`LaTeX-default-document-environment' in doc-string.
(LaTeX-environment): Use `LaTeX-default-document-environment'.

* style/beamer.el ("beamer"): Use
`LaTeX-default-document-environment' instead of
`LaTeX-default-environment'.

* style/letter.el ("letter"): Ditto.

* style/slides.el ("slides"): Ditto.