salt-minion/README.md: Discuss minion caching and sync_all

I needed the explicit sync_all with Salt v0.17.0.  It's possible that
this is a bug that has been (or will be) fixed in future versions.
salt-minion/Dockerfile.template: Install dev-python/git-python

Work around Gentoo's #485522 [1].

[1]: https://bugs.gentoo.org/show_bug.cgi?id=485522
salt-minion/Dockerfile.template: Add 'need logger' to salt-minion init script

Work around Gentoo's #502550 [1].

[1]: https://bugs.gentoo.org/show_bug.cgi?id=502550
salt-minion/Dockerfile.template: Bump to Salt v2014.1.0

With 17ca2ae (Merge pull request #9024 from cachedout/#8976,
2013-12-05, saltstack/salt#9024, [1]), Salt gained the ability to
report LXC-ness as a grain, which is useful if you're writing Salt
states that will run in Docker containers.  The series landed before
the v2014.1 tag and was released with v2014.1.0.

[1]: https://github.com/saltstack/salt/pull/9024