Couchbase-cli: Support resetting maxTTL for a collection

ns_server in the trinity release allows modifying a collection's maxTTL value to "bucket". This specifies the collection has no maxTTL value and should use the bucket's maxTTL if one has been specified for the bucket.

couchbase-cli should support this new option. Currently "bucket" isn't accepted for the --max-ttl argument. 

./couchbase-cli collection-manage -c localhost:9001 -u Administrator -p asdasd --edit-collection kylieScope.newcol --bucket kylie --max-ttl bucket
ERROR: argument --max-ttl: invalid int value: 'bucket'

The couchbase-cli folks can determine how best, syntactically, to specify it but when calling the rest api it should pass 'bucket'. e.g.

/pools/default/buckets/<bucket>/scopes/<scope>/collections/<collection> -d 'maxTTL=bucket' -X PATCH