proxyupdater should be started as first worker and block until SetUp is complete

[Michael Foord]

The proxyupdater is responsible for setting the process environment variables for proxy use, from the environment. Code (e.g. workers) that needs access to the network will need the proxy settings to be in place. The proxyupdater worker should be created first, and creation should block until the settings are in place (i.e. after SetUp has run).

When behind an enterprise proxy we're seeing intermittent failures with the charmrevisionupdater because of the race between proxy settings being in place and the charm revision updater doing it's work.