First part of addressing #1033 is ability to load credentials from the
credentials file(s).
This commit adds -i flag processing, i.e. a certificate can be fed via a
cert.file and used to login.
Functionality:
`teleport` binary now serves web assets from its own binary file.
Unless `DEBUG` environment variable is set to "1" or "true", in
this case it will look for ../web/dist (as located in github repo)
which can be used for development.
Design:
To avoid accumulating 3rd party dependencies with a ton of extra
features and licenses, this implementation uses minimalistic
implementation of http.FileSystem interface on top of the embedded ZIP
archive.
1. The assets are zipped into assets.zip during build process
2. assets.zip gets appended to the end of `teleport` binary
3. The resulting file is converted into a self-extracting ZIP
4. Teleport opens itself using the built-in zip unarchiver, and loads
the assets on demand.
Notes:
1. LOC is tiny (dozens)
2. RAM consumption is CONSTANT regardless of the ZIP size, about 500Kb
increase vs load-from-file, and most of it is linking zip archive
code from the standard library. Tested with a 20MB ZIP archive.
1. tctl auth export now dumps both user&host keys if --type key is missing
2. created fixtures for testing key imports: they're in
fixtures/trusted_clusters
3. configuration parser reads "trusted_clusters" files expecting the
output of tctl auth export
