How do I completely erase everything on my MacBook Pro (mid 2012) so that my husband can use it without having to clear one by one my iCloud account

What to do before selling or giving away your Mac - Apple Support

You should follow the directions in the above link rather than just erasing the drive.

You don't need one because your computer did not come originally with Yosemite. You need to install the original OS X version. Your husband can upgrade it using his Apple ID. He can't use the one held under your Apple ID. Below is what you do, essentially, once you have reached Step 5.

Install OS X Using Internet Recovery

Boot to the Internet Recovery HD:

Restart the computer and after the chime press and hold down the COMMAND-OPTION- R keys until a globe appears on the screen. Wait patiently - 15-20 minutes - until the Recovery main menu appears.

Partition and Format the hard drive:

1. Select Disk Utility from the main menu and click on the Continue button.
2. After DU loads select your newly installed hard drive (this is the out-dented entry with the mfgr.'s ID and size) from the left side list. Click on the Partition tab in the DU main window.
3. Under the Volume Scheme heading set the number of partitions from the drop down menu to one. Click on the Options button, set the partition scheme to GUID then click on the OK button. Set the format type to Mac OS Extended (Journaled.) Click on the Partition button and wait until the process has completed. Quit DU and return to the main menu.

Reinstall OS X: Select Reinstall OS X and click on the Install button. Be sure to select the correct drive to use if you have more than one.

Note: You will need an active Internet connection. I suggest using Ethernet if possible because it is three times faster than wireless.

This should restore the version of OS X originally pre-installed on the computer.