chore: Make LogSchema more optional

[Hoverbear]

Fixes #2110 .

PS C:\Users\Operator\CLionProjects\vector> cat .\test.toml
data_dir = "/var/lib/vector/"
dns_servers = []

log_schema.host_key = "instance_id"

[sources.source0]
max_length = 102400
type = "stdin"

[sinks.sink0]
healthcheck = true
inputs = ["source0"]
type = "console"
encoding = "json"

[sinks.sink0.buffer]
type = "memory"
max_events = 500
when_full = "block"


PS C:\Users\Operator\CLionProjects\vector> cargo run --no-default-features --features default-msvc -- --config test.toml
    Finished dev [unoptimized + debuginfo] target(s) in 0.90s
     Running `target\debug\vector.exe --config test.toml`
Mar 21 13:07:38.477  INFO vector: Log level "info" is enabled.
Mar 21 13:07:38.478  INFO vector: Loading configs. path=["test.toml"]
Mar 21 13:07:38.494  INFO vector: Vector is starting. version="0.9.0" git_version="v0.8.0-124-g838a80b" released="Sat, 21 M
ar 2020 19:50:16 +0000" arch="x86_64"
Mar 21 13:07:38.512  INFO vector::topology: Running healthchecks.
Mar 21 13:07:38.512  INFO vector::topology: Waiting for up to 3 seconds for sources to finish shutting down
Mar 21 13:07:38.513  INFO vector::topology: Waiting for up to 3 seconds for sources to finish shutting down
Mar 21 13:07:38.512  INFO vector::topology::builder: Healthcheck: Passed.
Mar 21 13:07:38.515  INFO vector::topology: Starting source "source0"
Mar 21 13:07:38.517  INFO vector::topology: Starting sink "sink0"
Mar 21 13:07:38.517  INFO source{name=source0 type=stdin}: vector::sources::stdin: Capturing STDIN
test
{"instance_id":"OBSIDIAN","message":"test","timestamp":"2020-03-21T20:07:40.652455500Z"}
Mar 21 13:07:42.278  INFO vector: Shutting down.

Added bonus:

PS C:\Users\Operator\CLionProjects\vector> cargo run --no-default-features --features default-msvc -- generate stdin//conso
le
    Finished dev [unoptimized + debuginfo] target(s) in 0.91s
     Running `target\debug\vector.exe generate stdin//console`
data_dir = "/var/lib/vector/"
dns_servers = []

[sources.source0]
max_length = 102400
type = "stdin"

[sinks.sink0]
healthcheck = true
inputs = ["source0"]
type = "console"

[sinks.sink0.buffer]
type = "memory"
max_events = 500
when_full = "block"

[binarylogic]

From planning: add behavior test that sets global option and tests this behavior.

[Hoverbear]

@binarylogic We can't use behavior tests for this until we do #1792