proxyquery: factor post_job cluster_config loading

There are 3 general cases for loading the cluster_config:

- POSTed tarball
- POSTed system.map/JSON job description
- POSTed script with shebang header

In order to reduce the overall size of `post_job`, these three cases are
now factored out into a separate method for each.
Add gzip to allowed tar mime types

For all intents and purposes, application/x-gzip is a type allowed in
requests, but it wasn't actually part of the `TAR_MIMES` constant (a
list) along with tar, gtar, and ustar. Also, several places in the code
that check if a given mime type is in the TAR_MIMES were including gzip
_additionall_, on the fly. This is more complicated than it needs to be.
Merge pull request #164 from larsbutler/factor-post_job

Factor `post_job`