puppetagent: do not use cache to rebuild image locally

This does not affect CI builds on wercker.
It does guarantee that local builds get upstream package updates.
If you run script/build.sh, it means the updates cascade to the
other images in this repo without having to specify --no-cache
within their docker files.
puppetdb: tune postgres shared buffers for kernel.shmmax 33554432

Postgres initdb command auto-tunes the database using settings of
the host on which initdb is run. If the docker image is deployed
on a host with smaller kernel.shmmax than the build host, then
postgres fails to start with:

    FATAL:  could not create shared memory segment: Invalid argument
    DETAIL:  Failed system call was shmget(key=5432001, size=37879808, 03600).

    HINT:  This error usually means that PostgreSQL's request for a
    shared memory segment exceeded your kernel's SHMMAX parameter.
    You can either reduce the request size or reconfigure the kernel
    with larger SHMMAX.  To reduce the request size (currently 37879808
    bytes), reduce PostgreSQL's shared_buffers parameter (currently
    4096) and/or its max_connections parameter (currently 103).

    If the request size is already small, it's possible that it is
    less than your kernel's SHMMIN parameter, in which case raising
    the request size or reconfiguring SHMMIN is called for.

    The PostgreSQL documentation contains more information about
    shared memory configuration.

This patch adjusts the `shared_buffers` parameter to an
empirically-derived value to work on host with limited kernel.shmmax.

Resolves https://github.com/jumanjihouse/docker-puppet/issues/15