Use start of string anchor regex for dir path ref

Previous to this commit, the pattern sent to `delete_lines` is an
(interpolated) string followed by a space, such as: `/nfs `, operating
on /etc/exports. Without the start of string anchor regex (`^`), the
string will match a different line that shares includes the string and
whitespace. Example:

`/shared `

will match:

`/shared ` (good)
`/opt/parallelcluster/shared ` (bad!)

Including a `^` forces the string to match from the beginning of the
line.

Author: Matthew Lesko

cookbooks/aws-parallelcluster-config/resources/manage_ebs.rb

@@ -184,7 +184,7 @@
     # unexport the volume
     delete_lines "remove volume from /etc/exports" do
       path "/etc/exports"
-      pattern "#{shared_dir_array[index]} "
+      pattern "^#{shared_dir_array[index]} "
     end
   end