ssh_auth: detect when source file has 0 valid keys

If an improperly-formatted file is used as the source file for an
ssh_auth.present state, the state incorrectly reports that all keys in
the file are present. This is because the functions that check the
source file only return keys that need to be changed, and with no valid
keys, there is nothing to change.

This commit detects when there are no valid keys found, and reports this
in the state output.

Additionally, since one of the ways that this edge case can be triggered
is if a public key from PuTTYgen is used, instructions have been added
to the ssh_auth.present docstring explaining how to obtain the
OpenSSH-compatible public key for the corresponding private key
generated by PuTTYgen.

Fixes #6277.
Merge pull request #6290 from terminalmage/issue6277

ssh_auth: detect when source file has 0 valid keys
Allow pip to install pre-release versions refs pypa/pip@4d5c5f8f
Merge pull request #6291 from mafrosis/pip_pre_releases

Allow pip to install pre-release versions
Limit pkg.latest to 1 refresh per run unless overridden

This fixes #6292 by moving the rtag check to the call to
pkg.latest_version, and never refreshing if/when pkg.install is run.
Add grains.append, grains.remove, and grains.delval methods to the grains module
Fix #5928

@basepi, this needs to be in 0.16.1
Merge pull request #6294 from kfdm/extra-grain-functions

Add append, remove, and delval to the grains module
Merge pull request #6293 from terminalmage/issue6292

Limit pkg.latest to 1 refresh per run unless overridden
Test if `--pre` option argument ends up in the resulting command line for `pip.install`.
cmd_iter_no_block did not pass on tgt to get_iter_returns(). That makes the
saltmaster always query ALL minions with saltutil.find_job because
get_iter_returns default to tgt='*'.
Return an empty dictionary instead of `ret` which is not yet defined at this stage.
Merge pull request #6295 from s0undt3ch/hotfix/pip-tests

Test if `--pre` option argument ends up in the resulting command line for `pip.install`
Merge pull request #6296 from felskrone/cmditernoblock_pass_on_tgt

cmd_iter_no_block did not pass on tgt to get_iter_returns(). That makes ...
Add file.missing state
Presumable fix for what's breaking Travis

************* Module salt.ssh
E0602:261,45:Single.highstate_seed: Undefined variable 'target'
Add missing colons in docstrings
Merge pull request #6301 from cvrebert/patch-3

Presumable fix for what's breaking Travis
Merge pull request #6302 from UtahDave/develop

Add missing colons in docstrings
yum: Allow 32bit pkgs in arches other than i686

The yum providers previously only supported 32bit packages that were for
the i686 architecture. This adds support for any iX86 arch.

Fixes #6299.