provider/azure: use new in-memory cert/key API in gwacl

A third-party contribution has just landed in gwacl to
allow users to pass the certificate and key as a []byte,
rather than requiring filenames to be specified. Juju
is updated to use this, removing the need to create
temporary files, simplifying the code.
Add comments about immutability/snapshots
Merge pull request #872 from axw/azure-cert-no-tempfile

provider/azure: use new in-memory cert/key API in gwacl

A third-party contribution has just landed in gwacl to
allow users to pass the certificate and key as a []byte,
rather than requiring filenames to be specified. Juju
is updated to use this, removing the need to create
temporary files, simplifying the code.

http://reviews.vapour.ws/r/136/