Add parameter for master's port in agent

Hello dear sir,

we're running in an environment which prevents us from running puppet in the standard port 8140. This parameter would allow to use the custom port.
There needs to be validation. Suggest using is_integer()

https://github.com/puppetlabs/puppetlabs-stdlib#is_integer.

There needs to be spec tests that show that by default the line is not in the config file. If set to 
a valid integer => it is in the config file
a non string => it throws an error
a string that is not an integer => it throws an error