sqlplus works specifying the parameters but fails when using tnsnames.ora
I could fix the problem. I changed my TNS_NAMES.ORA removing ADDRESS_LIST and leaving like this:

ORCL=
(DESCRIPTION =
  (ADDRESS=(PROTOCOL=TCP)(HOST=msoracle.ccccc.us-east-1.rds.amazonaws.com)(PORT=1521))
  (CONNECT_DATA=
  (SID=ORCL)
  )
)


After I created a new file sqlnet.ora and added this:

NAMES.DIRECTORY_PATH = (TNSNAMES)


I also (just in case), renamed both files to lowercase:

[franklin@ip-10-148-17-253 admin]$ ls -ltr
total 8
-rw-r--r-- 1 root root 151 Aug  5 13:08 tnsnames.ora
-rw-r--r-- 1 root root  34 Aug  5 13:08 sqlnet.ora


So now I can connect just doing this: 

sqlplus myuser/mypassword@ORCL