vtep: Don't try to both distribute and distclean the manpage.

Since the initial checkin of the vtep code, the manpage has been both
distributed and distcleaned.  That's contradictory.  This commit fixes it.

This commit also moves the vtep manpage from the source directory to the
build directory.  There's no real reason for the manpage to be in the
source directory, and it can't be if it's not distributed (because "make"
is not supposed to update the source directory in a freshly untarred
source distribution, and doing so breaks "make distcheck").

Found by "make distcheck".

Signed-off-by: Ben Pfaff <blp@nicira.com>
Acked-by: Joe Stringer <joestringer@nicira.com>