This is used when sending execution requests, to specify which instance of the
execution server to use.

Partial fix for #627.

Adds instance name support for the remote execution storage service.

Adds the 'instance_name' parameter, which may be None, to most GRPC
calls in the CASCache object. ByteStream requests already have
instance_name supplied in the resource name, so do not need the
parameter.

Closes #627.

Remote-execution instance configuration support

Closes #627

See merge request !952

Fixes issue #740

Shell --build has optional buildtrees

Closes #740

See merge request !986

README.rst: Add license badge

See merge request !1014

Adds basic functionality for `bst fmt`, formatting the yaml files of
elements

TODO:
* Weird issue in yaml dumping - see #767
* Add a check option, which just checks if format correct
* Add a canonical order of top-level nodes on a per project basis

Allow authors of plugins to utilise the Plugin.keyorder attribute to
define an order for the sub-headers in config. Modify `bst fmt` to
modify the yaml into that order. Add canonical keyorder for every plugin
already in core.

In terms of implementation, this adds a custom yaml dumper subclassed
from ruamel.yaml's RoundTripDumper, which will dump a dict in the order
defined in the keyorder attribute. The Plugin class is given a keyorder
attribute, which defines the order in which keys should be dumped. The
element and source classes add the common fields used in them, so as to
minimise work for plugin authors. Plugin authors should add their custom
keys at configure time.

This causes some stripping of comments, which seems to be due to a bug
in the ruamel.yaml RoundTripDumper, as that also strips the same
comments. It also will remove blank spaces, again probably due to
limitations in ruamel.