Give flux bootstrap the extra components flag

This commit adds a flag for supplying extra components to bootstrap
(and its subcommands), to match the one for `flux install`.

Since the bootstrapComponents global is used in a few places, I made
it a func and renamed the variable. For consistency, I also renamed
the var used in install.go.

Lastly, so that the flag sorts next to `--components`, I changed it to
`--components-extra` in both commands.

Signed-off-by: Michael Bridgen <michael@weave.works>