This adds an env var-driven configuration for the filesystem path
in which kustomize should expect its plugins (including airshipctl)
to be. The value defaults to a subfolder off the `.airshipit` folder.
The config is persisted as a singleton rather than a member of
AirshipCTLSettings (or the Config) because
1. the settings object would have had to have been passed around
a couple dozen additional files/functions,
2. it's reasonable to expect the plugin location to be consistent
across threads in a multi-threaded, airshipctl-as-library context.
Settings_test.go was moved in to an environment_test package to avoid
a circular import dependency.
Change-Id: Icdd21bd3687ef42492e388af982d7b490af3eff3
4e64e74b6f