Make the environment variable name more explicit

[moul]

[moul]

@QuentinPerez, can you list current environment variables ?

[QuentinPerez]

Currently we have:

• SCW_COMPUTE_API
• SCW_ACCOUNT_API
• SCW_METADATA_API
• SCW_MARKETPLACE_API
• SCW_VERBOSE_API
• SCW_TLSVERIFY
• SCW_SENSITIVE
• SCW_COMMERCIAL_TYPE
• SCW_TARGET_ARCH
• SCW_TARGET_REGION
• SCW_NOCHECKVERSION
• SCW_GATEWAY
• SCW_GOTTY_URL
• scaleway_api_endpoint
• DEBUG
• exec_secure

[moul]

Ok cool, so basically we can let DEBUG as it is, remove scaleway_api_endpoint and rename exec_secure to SCW_SECURE_EXEC

[moul]

See #312