config: Add a new option connect_timeout

connect_timeout can be used to increase the time Spack waits for the
server to answer. This can be used to work around slow connections or
servers.

Fixes #14700

etc/spack/defaults/config.yaml

@@ -75,6 +75,12 @@ config:
   misc_cache: ~/.spack/cache
 
 
+  # Timeout in seconds used for downloading sources etc. This only applies
+  # to the connection phase and can be increased for slow connections or
+  # servers. 0 means no timeout.
+  connect_timeout: 10
+
+
   # If this is false, tools like curl that use SSL will not verify
   # certifiates. (e.g., curl will use use the -k option)
   verify_ssl: true