Add password length validation - take 2

[hwwhww]

Address #99
Replace #118

I haven't figured out how to test hidden input (password) with the test script, but #123 gave me a hint of utilizing click callback function. This is a workaround of #118.

Example output

1. enter the length=3 password and confirmation -> got rejected
2. enter the length=5 password -> got rejected
3. enter the length=8 password and confirmation -> got accepted

. venv/bin/activate && python ./eth2deposit/deposit.py
Please choose how many validators you wish to run: 1
Please choose your mnemonic language (czech, chinese_traditional, chinese_simplified, english, spanish, italian, korean) [english]:
Please choose the (mainnet or testnet) network/chain name (mainnet, witti, altona, medalla, spadina, zinken) [mainnet]:
Type the password that secures your validator keystore(s):
Repeat for confirmation:
Error: The password length should be at least 8. Got 3. Please retype again.
Type the password that secures your validator keystore(s):
Error: The password length should be at least 8. Got 5. Please retype again.
Type the password that secures your validator keystore(s):
Repeat for confirmation:

Compare with #118

	#118	This PR
1.	--password flag is disallowed	--password flag is allowed
2.	Validate the password strength once the user enters the password	Validate the password strength after it passing the confirmation. After that, it validates the strength once the user enters the password. (another while loop)

I do want to avoid (1), but (1) helps us writing the test script. 😓

[CarlBeek]

I think this is a better solution overall than the one presented in #118. 👍

I do with the order was verify length then check for matching passwords, but this may be an example of perfect being the enemy of the good.

[hwwhww]

edited PR description:

#118: --password flag is disallowed
This PR: --password flag is allowed