Properly handle bootstrap script in seed and lxc modules.

This is related to be605bc81a43879c15f5b026f5d2529a9b008c7e and #14865.
i am aware of them, the lxc salt cloud driver used that code at first, look also to the utils.cloud changelog to see that i am not a stranger in it ;) , but i did not managed to come to a consensus with @mgwilliams, so i reused and adapted to factorize the lxc exec module and the salt cloud driver.

lxc.bootstrap+seed is a redundant part and is working as expected with this fix, we should just stabilize it ;)