Support `--timeout` flag

[nvthongswansea]

Add --timeout flag so we can set timeout when dealing with gs resources. E.g:
./gscloud server --timeout 10m

[bkircher]

Besides the global flag, we could also expose this in the config. Would allow me to specify different timeouts per project / datacenter once in the config file.

[bkircher]

@nvthongswansea free and interested to work on this? Otherwise I'll move this out of the milestone.

[nvthongswansea]

@nvthongswansea free and interested to work on this? Otherwise I'll move this out of the milestone.

I will work on this in the future.