Do not validate identity flag to allow for sha1 values

[gsabran]

When several code signing identities have the same name, codesign will fail:

security find-identity -v -p codesigning
 1) 3F60B8D9578EAEF1E13A7EEC1394D4CE34139555 "Developer ID Application: App Inc (FR57J8KL90)"
 2) 7952E8BC89E7BF5FCED31AF352783B7A34FADC0D "Developer ID Application: App Inc (FR57J8KL90)"
 
 
create-dmg 'build/myGreatApp.app' 'build/' --identity 'Developer ID Application: App Inc (FR57J8KL90)'
✖ Code signing failed. The DMG is fine, just not code signed.
Developer ID Application: App Inc (FR57J8KL90): ambiguous (matches "Developer ID Application: App Inc (FR57J8KL90)" and "Developer ID Application: App Inc (FR57J8KL90)" in /Users/me/Library/Keychains/login.keychain-db)

codesign will however work if you give it the matching sha1 for the identity to use.

This change allow to use sha1 as well as identity name by removing the validation for the identity flag that allows for only the name. Not validating seemed fine (instead of validating either possibilities) since codesign will fail when the identity is not valid:

create-dmg 'build/myGreatApp.app' 'build/' --identity '3F60B8D9578EAEF1E13A7EEC1394D4CE34139555'
ℹ Code signing identity: Developer ID Application: App Inc (FR57J8KL90)
✔ Created “myGreatApp 1.0.0.dmg”

create-dmg 'build/myGreatApp.app' 'build/' --identity 'bad-identity'
✖ Code signing failed. The DMG is fine, just not code signed.
bad-identity: no identity found