[connman] gdhcp: adjust discovery/request timeout and retry values

Some dhcp servers are acting lazy (eg. Buffalo WBMR-G125)
therefore 3 second timeout value for discovery or request
is not enough. Adjusting the timeout value from 3 seconds
to 5 will fix the problem together with adjusting the
retry value not to increase the total time for waiting
for getting the lease.
Merge pull request #103 from tigeli/dhcpdiscovery

[connman] gdhcp: adjust discovery/request timeout and retry values