In certain scenarios the default poll interval of 5s used by
`openstack stack create --wait` command is too short.
Setting the poll interval was supported in heat CLI with
`heat stack-create --poll N` but is missing in OSC plugin.
This patch adds an optional argument `--poll N` (N defaults to 5) to the
`openstack stack create` command.
Change-Id: Id279d92ea890032f280e453b795ede2818ffbb8c
Story: 2004863
Task: 29106
033511c291