feat(helm): add 'helm fetch --prov' to fetch prov info

fix(helm): give different error if key is not private

fix(helm): add trimSuffix to helper functions

fix(helm): fix broken --values flag

Previously, a "not found" error was returned if a key exists, but is not
a private key. Updated the error to better indicate the case.

fi(helm): add more tests for plugins

This fixes the 'helm upgrade --values', which I broke when fixing
'--set'.

Closes #1631

feat(tiller): update sprig to 2.7.0

This adds shasum, int, and float64 functions, and fixes quote escaping
for 'quote'.

Closes #1524

fix(helm): fix prompt error on Windows build

This backs out an earlier change that removed an int type assertion.

fix(hel): fix plugin format on tests

This adds tests for setupEnv, and also updates some documentation to
mirror the current state of plugins.

fix(helm): handle errors when plugin command is not found

fix(*): misc linter issues

fix(helm): add missing line ending on list output

Feature/repo url trailing slash

fix(ci): do not push canary image on release

chore(*): bump go1.7.3

fix(helm): write yaml without exec permission

feat(helm): Add Files.Glob method to permit file organization

Test plugins were in the original format. Updated to current format and
fixed the tests to stop silently ignoring the broken test.

If a 'command:' is not found for a plugin, it will not result in an
ExitError, but in a PathError. This prevents that condition from
panicing.

Closes #1609

Previously, there was no way to get provenance data unless you were
verifying it. This allows `fetch` to get the provenance data, but not
perform any verification. Adding this will allow external plugins
to perform verification against other sources, like Keybase.

fix(helm): add no-hooks to upgrade

Mark disable-hooks as deprecated.

Closes #1575

fix(helm): pass the no-hooks flag during upgrade

fix(helm): improve URL comparison logic