A user reported that the api-get command was failing with an error

attempting to run without explicitly listing the server name in the

command-line arguments or .reviewboardrc. In this case, they were

using the reviewboard.url perforce counter to define the server name

for their repository.
When there's no explicit server name configured, we will initialize the

repository client(s) in order to use them to detect the server name.

Unfortunately, the api-get command didn't include the repository

command-line arguments (because why would it), and the code path that

does this was assuming that it existed.
This change fixes it so we only conditionally look for the value of the

--repository-type argument inside the argparse results. If not found,

we'll then proceed through our normal repository detection code.

Set up a perforce server that defined the reviewboard.url counter, and

then ran rbt api-get users from within a perforce client. Saw that it

correctly determined my Review Board host to use, rather than crashing

with an AttributeError.