Support HTTP proxies for repository connections

[soenkeliebau]

The agent currently does not honor the HTTP_PROXY environment variable and offers no command line parameters to configure a proxy either. For the connection to Kubernetes a proxy is not supported, so that is out of scope, but for the repository downloads we should support specifying a proxy in some way.

Options that come to mind are:

• use HTTP_PROXY environment variable (and NO_PROXY) - tbd if upper or lower case is correct here, it seems that lower case is the norm..
• offer an explicit command line parameter
• configure proxy in repo object in Kubernetes (this breaks if some machines need a proxy and some don't)

[soenkeliebau]

Blocked by packaging discussion