feat(Landscape): Hostagent URL moved to Landscape config file

[EduardGomezEscandell]

The landscape client config currently needs at least three fields:

[client]
url = https://landscape.canonical.com/message-system
ping_url = http://landscape.canonical.com/ping
account_name = dummy

There is another registry key right now called LandscapeAgentURL which is used for the Windows-Agent gRPC connection. It would make more sense to put this information in the config as well, for example:

[host]
url = https://landscape.canonical.com/hostagent

[client]
url = https://landscape.canonical.com/message-system
ping_url = http://landscape.canonical.com/ping
account_name = dummy

The host section is removed before writing the file to /etc/landscape/client.conf.

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In the future, we'll have to change the default value of host/url. It defaults to empty string both before and after this PR. We'd want it to default to Canonical's URL. This has been captured in Jira card 1687.

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UDENG-1644

[didrocks]

Some questions, overall good, but I think we can make administrator’s life a little bit easier :)

[didrocks]

Excellent!

[EduardGomezEscandell]

I was too excited to merge and didn't squash commits :S