Multiple YubiKeys support

[renatav]

Description (e.g. "Related to ...", etc.)

This PR includes several major reworks/new features:

• Key names defined in config files when creating a new repository or adding a new role are added to metadata files. This is possible following the transition to the newest version of TUF and securesystemslib. These names are loaded from metadata files when a repository is initialized and used when asking the user to insert a specific YubiKey.
• When signing metadata, we now iterate over all inserted YubiKeys. The users will no longer have to remove and insert YubiKeys several times.
• Add a PinManager class. Instances of this class can be passed into API functions, enabling reuse of already entered pin. Removed all code that stored pins in global dictionaries, as this is less secure.
  This will allow creation of scripts like this one:

pin_manager = PinManager(auto_continue=True)
pin_manager.add_pin(25924217, "123456")
update_metadata_expiration_date("auth1/auth1", pin_manager, roles=["root"], keystore="keystore")
update_metadata_expiration_date("auth1/auth1", pin_manager, roles=["targets"], keystore="keystore")

Of course, a PIN should not be specified in this manner when using production YubiKeys. If the PIN manager is empty, the first time a YubiKey is needed, TAF will prompt the user to enter the PIN. The script could also ensure that all PINs are securely entered before calling any of TAF's API functions

Signing after this rework will look like this:

Scenario 1 - no YubiKeys inserted initially, the user inserts one YubiKey first and then the second one later

Sign targets using YubiKey(s)? [y/N]: y
Please insert targets (userYK1, userYK3, user1) YubiKey(s) (threshold 2) and press ENTER
Enter PIN for userYK1:
Confirm PIN for userYK1:
Please insert targets (userYK3, user1) YubiKey(s) (threshold 2) and press ENTER
Enter PIN for userYK3:
Confirm PIN for userYK3:
Threshold of targets keys reached. Do you want to load more targets keys? [y/N]: n

Scenario 2 - no YubiKeys inserted initially, the users inserts both YubiKeys at the same time:

Sign targets using YubiKey(s)? [y/N]: y
Please insert targets (userYK1, userYK3, user1) YubiKey(s) (threshold 2) and press ENTER 
Enter PIN for userYK1:
Confirm PIN for userYK1:
Enter PIN for userYK3:
Confirm PIN for userYK3:
Threshold of targets keys reached. Do you want to load more targets keys? [y/N]: n

Here are config files used to create repositories and specify addition of a new role:

keys-description.json
keys-description2.json
keys-description3.json
keys-description4.json
role-config.json

Fix #244

Code review checklist (for code reviewer to complete)

• Pull request represents a single change (i.e. not fixing disparate/unrelated things in a single PR)
• Title summarizes what is changing
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• Tests have been included and/or updated, as appropriate
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• Changelog has been updated, as needed (see CHANGELOG.md)