Fix accounts-v2 deposit and related bugs

[dv8silencer]

What type of PR is this?

Bug fix

What does this PR do? Why is it needed?

Issue #7164 outlines a bug with depositing using cli:
(from @danielschonfeld )

./prysm.sh validator accounts-v2 deposit --wallet-dir /home/myuser/.eth2validators/whale_prysm --eth1-keystore-utc-path /home/myuser/.ethereum/goerli/keystore --eth1-keystore-password-file /home/myuser/mypasswdfile --deposit-all-accounts --http-web3provider http://localhost:8546

Creating this output

[2020-09-02 11:40:04] FATAL accounts-v2: Could not send validator deposit(s): read /home/myuser/prysm: is a directory

In the path to resolving this bug, its cause and other bugs were discovered and are addressed by this PR:

• validator/accounts/v2/accounts_deposit.go line 76: Based on the already written-comment in code ("If the user has selected to deposit all accounts, we skip this part.") and review of the logic, the wrong flag was being checked in the if; it should be DepositAllAccountsFlag
• validator/accounts/v2/accounts_deposit.go line 133: This logic is meant to capture cases of hasPrivateKey || hasEth1Keystore but only deals with what should happen if Eth1PrivateKeyFileFlag has been provided (hasPrivateKey). So if a user submits a path to an Eth1Keystore, this part of the logic is hit and soon returns from the function, but that flag and its content is never regarded.
• validator/accounts/v2/prompt.go line 64: The codepaths that lead to the function call inputWeakPassword() expect it return a path, not read the content into the return variable. While testing this, you can craft the command line arguments so that the program ends up trying to look for a file such as /path/to/a/path/(contentOfFile) because the function returns the content of the file, not the path. I changed it so it returns the path.

Which issues(s) does this PR fix?

Fixes #7164

[codecov]

Codecov Report

Merging #7219 into master will decrease coverage by 0.00%.
The diff coverage is 29.41%.

@@            Coverage Diff             @@
##           master    #7219      +/-   ##
==========================================
- Coverage   61.68%   61.68%   -0.01%     
==========================================
  Files         412      411       -1     
  Lines       32969    32785     -184     
==========================================
- Hits        20337    20223     -114     
+ Misses       9726     9682      -44     
+ Partials     2906     2880      -26