Introduce environs/bootstrap.BootstrapParams

The user-facing Bootstrap method (i.e. as
used by cmd/juju) will take different parameters
than the provider-specific one (Environ.Bootstrap).
Thus, we introduce a new parameter struct that is
currently identical to environs.BootstrapParams,
but will be changing imminently.
Make environs/bootstrap responsible for tools location

environs/bootstrap now finds available tools, augmenting
or substituting for locally buildable/uploadable tools
as necessary (auto upload) or as specified (--upload-tools).
Environ.Bootstrap is now provided with a list of available
tools, and is not responsible for syncing tools itself.

--upload-series and --series will be obsoleted: we will
upload a single series, and then use that in the metadata
for each supported series.
Explode uploaded tools for each supported series in bootstrap
Fix tests
environs/cloudinit: Wrap tools download inside cloud init in a retry loop

Forward port of https://github.com/juju/juju/pull/598
environs/bootstrap: rework findAvailableTools
Revert changes to mount tools storage in containers

This conflicts with provider storage removal.
Machines will retrieve tools from the API
server in a follow up PR.
cmd/juju: update --(upload-)series message/help
environs/bootstrap: tweak comment
environs/bootstrap: more coverage in findAvailableTools
cmd/juju: fix test on i386 (don't assume amd64 tools)
provider/common: set default-series in mock config
Merge pull request #600 from axw/bootstrap-upload-tools-no-storage

Make environs/bootstrap responsible for tools location

environs/bootstrap now finds available tools, augmenting or substituting
for locally buildable/uploadable tools as necessary (auto upload) or as
specified (--upload-tools). Environ.Bootstrap is now provided with a list
of available tools, and is not responsible for syncing tools itself.

--upload-series and --series are obsoleted: we upload a single series,
and then clone that for each supported series. When we store the tools
catalogue in state, we will simply write an entry for each series that
points to the same tools tarball.