Install repository key from repo, not HTTP

Jenkins unfortunately don't provide their repository key from an HTTPS
conneciton. Installing such a key without authentication is not good as
it is wide to to man-in-the-middle attacks.

This commit makes ansible install the key directly from the repository.
If the key changes, we will have to verify and update it here, but this
feels like a justifiable change.
Don't run apt-get update before each dependency

There's no need `apt-get update` before installing if we're just using
`state=installed`.

Removing these significantly speeds up the entire process.
Add 1 day cache timeout for Jenkins package

We want to the latest Jenkins (`state=latest`) so we do `apt-get update`
before attempting to install. It's overkill to do this more than once
per day, however, so set a cache time of 86,400 seconds.

(Also fix parameter name from `update-cache` to `update_cache` as the
the docs)
Ensure we apt-get update after adding Jenkins repo

Although it's the default behaviour, it's good to make sure we actually
do an `apt-get update` after adding the new Jenkins repository.

Also remove some dead (commented) code.
Merge branch 'install-apt-key-from-repo' into development