Fixes #900: Removes '0' from the list of valid ports

[tjmcs]

Fixes #900

Summary of changes:

• Removes '0' from the list of valid ports that the RESTful API can be started on (as such, since a non-zero port must be specified or the default port used, the RESTful API will not have a port dynamically assigned when snapd starts up so there is no need to display the port that was chosen in the log file)
• Refactored how the command-line flags are set so that the command-line flags for each package are set locally in a flags.go file for that package rather than having the command-line flags for the options from some packages set in a package-level flags.go file and while the command-line flags for the options for other packages are set in the snapd.go file
• Modified the code that sets the command-line flags so that those that are displaying an example value display the default value for that option in the --help output for snapd (rather than displaying values like '0' for command-line options like the --api-port)

Testing done:

• Tested by attempting to start snapd with the port set to zero in the global configuration file and observing the error that results, then by starting it with a non-zero port in that same configuration file and observing the successful startup. Also ensured that the --help output for snapd could be generated successfully with the refactored command-line option code in this PR.

@intelsdi-x/snap-maintainers

[jcooklin]

LGTM