Fix `morph help-extensions` returning no items

We were passing a nonsensical value for the 'kind' parameter so it
would always return an empty list.
Fix `morph help-extensions` run outside a workspace

This command would use the sysbranchdir.open_from_within() function
call to search for a system branch, which traverses all subdirectories
looking for a directory. This is useful behaviour, but if the user ran
`morph help-extensions` in / it'd traverse their whole filesystem before
returning, which is stupid. This change means it only does the traverse
if it already detected a workspace.
Add missing types to the docstring in the DeployPlugin.deploy() function
OpenStack write extension documentation
ssh-rsync write extension documentation
rawdisk write extension documentation
initramfs  write extension documentation
Add a reference to write.help file
Merge branch 'sam/fix-help-extensions'

Reviewed-By: Pedro Alvarez <pedro.alvarez@codethink.co.uk>
Reviewed-By: Emmet Hikory <emmet.hikory@codethink.co.uk>
Merge remote-tracking branch 'petefoth/pf-document-extensions'

Reviewed-By: Adam Coldrick <adam.coldrick@codethink.co.uk>
Reviewed-By: Sam Thursfield <sam.thursfield@codethink.co.uk>
deploy: Note that some of the extensions listed live in definitions.git

The user should be aware of this because if they aren't building
baserock:baserock/definitions or a repo forked from it, those extensions
won't be available.

Also fix some long lines that I seem to have failed to commit already.