Update default_verify() to correctly respect the VAULT_SKIP_VERIFY env var

[randombugfixes1234]

Hi! The vaultrs crate currently does not correctly handle this environment variable. Expected behaviour (reference to Vault docs) is as follows:

• VAULT_SKIP_VERIFY=true : TLS verification is skipped, i.e. verification is not performed
• VAULT_SKIP_VERIFY=false : TLS verification is not skipped, i.e. verification is performed
• VAULT_SKIP_VERIFY is not set : TLS verification is performed by default

The current behaviour is correct when the env var is not set, but is inverted in the case that it is. In other words, currently using VAULT_SKIP_VERIFY=true with vaultrs means that verification is performed (and vice versa). The unit tests are also backwards.

This PR corrects default_verify() to handle this properly and updates the unit tests for consistency.

[stormshield-gt]

Thanks for the PR, I guess it supersedes #94